How to know how much CPU and RAM I need for new computer

To answer your question about performance, it really depends on what you're wanting to do.

CPU: To put it simply, CPU speed defines how quickly your computer can process data. As you're asking the question in a Mac forum I'll assume you want to by a mac. The most basic MacBook Air laptop has 1.7GHZ dual core processor, which means it has two 1.7GHZ processors. For basic computing this is more than enough, but if you wanted to do something a bit more labour intensive, say photo editing or graphic design or high end gaming, it'd probably be a bit slow. If you look at the slightly more advanced machines apple has on offer like the macbook pros or the iMacs then the processors are more in the 2.5GHZ - 2.9GHZ range, this is plenty for most high end applications. Finally the top of the range machines are the Mac Pros. They have quad core, 8 core and 12 core options upward of 3.2GHZ. These are great machines if you have the money and will make photoshop run a little faster, but not really a necessity unless you want to do some really high end stuff like Animation or complex video editing.

Memory: Memory consists of what can be considered your long term memory (your hard drive) which is sometimes called "storage" and your short term memory or RAM (stands for Random Access Memory if you're geeky enough to be interested) which is what the term "memory" is normally referring to. Ram controls how much your computer can store in the short term to process. Most basic PC's and the most basic macbook airs come with 2 gigs of ram nowadays which is more than enough for basic applications. However, again if you're looking to do photo editing or graphic design or online gaming then you really should be looking at between 4g and 8g otherwise your computer won't have enough memory to process lots of different functions at the same time. And of course, if you're a budding Peter Jackson about to make the next lord of the rings film and you need to be able to render an army of mediaeval monstors, then the mac pro ranges from 16g to 64g of ram.

Hope this answers your question better.

Because OSX manages memory (as does Windows) it is often difficult to tell if performance issues are caused by memory limits. Technically, you really want the OS to capture all the available memory and allocate as needed, in which case, you would see all of the memory being 'used', even though there may be memory available but not allocated by the OS.

This is somewhat analogous to what is happening in OSX and Win7+. However, Apple has made it confusing in the past with many memory classifications, like Wired, Swap, Real, etc.

Luckily, you are running Mavericks, and a new feature of Mavericks is the Memory Pressure graph, described in the Apple support document here: http://support.apple.com/kb/HT5890

Basically, if the memory graph shows Red, you need more RAM, if its Green, you are good. I suggest reading the article for more details, but the Red, Yellow, Green is a great visual tool to help guide where you might be seeing problems.

Depending on your model, I have personally found Macs suffer more from hard drive access speed than memory limits, so an SSD, if your iMac does not have one, has a huge impact (and unfortunately quite a bit of work to put into your iMac)