Mixing RAM sticks with different CAS Latency and timings

Voltage, Latency and timings are not important factors.

... Well, Voltage is very important - but if you get standard grade desktop commercial components, they should all meet the standard. It is just some overclocked or special server memory uses different voltage, however they are usually more expensive and warn you.

Latency and timings, there are rare instances where it is not possible to have differences - usually some chipsets/motherboards prevent this, however, in most instances you will just get a warning when you start the machine and it will operate at a low speed (or even a penalty speed where it is slightly lower than the lowest - only seen this once on a American Megatrends BIOS years ago).

As it is part of your question - it shouldn't cause any damage to the machine. The machine is designed for the memory socket. At worst you will get a beep warning and/or the memory will run slow. I cannot imagine any irreversible damage. Even if you had memory requesting higher voltage, I could imagine the memory stick failing before the motherboard - but don't test it or quote me on that!

As for buying, my guide will always be to try and buy a matching / identical second stick as this enables dual (or higher) channel memory support, if your motherboard supports it. This mode can (under some circumstances) dramatically increase memory performance.

Reasons to go for two:

Main reason considered is for the extra throughput of the dual channel (accessing both RAM modules at once). The actual improvement from this is a subject of some debate though e.g. http://www.tomshardware.com/reviews/PARALLEL-PROCESSING,1705-15.html

As expected, the performance difference between single channel and dual channel DDR2-800 memory using an up-to-date Core 2 Duo system Compare Prices on Core 2 Duo Processors is little to nil, depending on the benchmark - most tests show differences, but they are really small. For games and enthusiast PCs, we recommend sticking to high-performance dual channel RAM, because the memory is one of those components that you want to perform best for a smooth experience. For regular applications, though, it doesn’t really matter much whether you run single or dual channel. Two 1 GB DIMMs typically are cheaper than a single 2 GB module, but a single DIMM will reduce your power consumption by several watts (which might just be more interesting than it is important).

Cost is also potentially a factor as two smaller DIMMs are often cheaper than the single large DIMM (as mentioned in the quote above)

Reasons against are those of extra heat and system load; and also the upgradability point that you raised initially.

There's debate on similar lines in the following SU question: Is it better to have four times 2 GB or two times 4 GB of RAM?