Does upgrading RAM with different Speed, affect the pc or affect all the RAM

There is a very simple way to demonstrate the timing of memory in very practical terms everyone will understand. The Megahertz and Gigahertz of clock speeds and bus speeds can seem a bit opaque if you do not have an electronics background.

The first thing to consider is the actual clock speed. The clock speed is effectively the number of times per second the computer can conduct an operation. The operations are usually read or write in the case of memory. The clock speed and synchronization is needed so all of the electronic components know when to listen for the electrical signal that represents a 1 or a 0. If either side is early or later talking or listening then there is a high likelihood of an error determining the correct state of the bit in memory.

Second lets abstract this as if it were a phone call. Imagine we are both on a phone connected directly to each other. We have a metronome that is clicking once every five seconds and every time it clicks we take turns speaking. We are exchanging information back and forth. We express the information in a predetermined way which is to scream over the line when the metronome clicks to represent a 1 in memory and silence to represent a zero.

Now that the example is laid out I can use this to demonstrate a few things about the way ram functions. The protocol in this example is we take turns every time the metronome clicks. If either of us miss one of the clicks of the metronome we find ourselves out of sync. Synchronization errors are effectively expressed when the two of us are not talking and listening at the right moments. If you start listening at just a milisecond after I stopped yelling, then you would eroneously interpret that to be a 0 state. They call this jitter. The worse the two sides get out of sync, the more pronounced the number of state determination errors will occur.

The clock speed is needed to allow the motherboard and the memory to correctly exchange state information with one another. The clock speed of the memory is more or less equal to the speed at which it is capable of reading/writing data to RAM.

The reason there is such a variation in speeds of memory modules is because over the last several years, materials science has developed lower power memory that is capable of maintaining a greater number of reliable state interrogation points per second effectively making the memory faster. The time it takes the electrical signal in the wire to go from a complete 0 to a complete 1 is called transient time (Also referred to as low and high states) When reading/writing memory, the closer the read/write is to the clock sync pulse, the more likely it is the read/write will be successful. The closer it is to the mid point between clock pulses, the more likely that the read/write will be unsuccessful.

Most average users do not get into the nitty gritty details like this, but if you are brave and have designs to overclock a computer or crank up the bus speed then you probably care much more about this sort of thing. You can often times get greater speed out of electronics but the side effect is more heat and more errors. The heat is a function of the increase in the number of operations taking place and the errors are usually directly related to the particular performance characteristics of the semiconductor material in the memory. The speed rating of memory is more or less just performance metric the memory is designed to achieve with a acceptable amount of read/write errors.

I hope this answers your question....

An article from Toms Hardware does a very good job at explaining RAM timings

• CAS, usually expanded as column address strobe (or sometimes as column address select), which refers to the column for some physical memory location in an array composed of columns and rows of capacitors used in dynamic random access memory (DRAM) modules (of which all three types of RAM in this guide are sub-types). CAS latency generally appears first in timing sequences for RAM and indicates the number of clock cycles that elapse between when the memory controller instructs the memory module to access a particular column in its current row, and when such access produces the data that resides there.
• Trcd or tRCD, usually expanded as RAS to CAS delay, where RAS is expanded as row address strobe, where R refers to the row for a physical memory location in an array composed of columns and rows of capacitors used for DRAM modules. This value specifies the number of clock cycles between a Row Address Strobe (RAS) and a CAS, and represents the row address to column address delay for a memory module.
• Trp or tRP, usually expanded as RAS precharge, which represents the number of clock cycles required to end access to the current row of memory, and commence access to the next row of memory, so that tRP = time for row precharge.
• tRAS or Tras, usually expanded as RAS access time measured by the number of clock cycles needed to access a certain row of data in DRAM between the initial data request and the precharge command required to commence the next memory access. By definition, tRAS must be greater than or equal to the CAS plus the tRCD, plus an additional two cycles, to leave time for accesses to complete, as they read or write multiple bits of memory, which DDR (2 bits), DDR2 (4 bits), and DDR3 (8 bits) all do in lesser or greater numbers.

RAM memory timings usually appear as sequences of four numbers separated by dashes, as in 5-5-5-15. This indicates that the CAS, tRCD and tRP values are all equal to five clock cycles, and that the tRAS value is equal to 15 clock cycles. The smaller the numbers that appear in these sequences, the tighter memory timings are said to be. Likewise, larger numbers are said to indicate looser timings. Simply put, lower latency costs more, tighter timings cost more and the combination of the two costs the most where memory is concerned.

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I would go with the tighter timings they tend to be faster and the 166mhz difference will not even be noticeable. In fact they are both so close that you would not notice any difference in speed. The one with the higher Mhz rating might be best for over clocking since it will give you a little head room since modern Intel and all AMD chips have integrated Memory controllers so as you up the FSB the you will also be over clocking the RAM. But again this is all personal preference depending on what you will do with your system. Either one will be fine if you are just building a stock system that you don't plan on overclocking.