1.) Update you graphics card drivers. This really helped for me, as I was also experiencing lag, but then I upgraded my drivers and all was well.
2.) Clean your PC of crap. Crap can build up on your computer, slowing it down and using resources. Here is a program that will clean the crap from your PC:
Download | The PC Decrapifier
3.) Update your OS. Updating your operating system can really help. I have XP, and I sued to have SP1. I upgraded to SP3, and my speed increased dramatically.
4.) Clean your PC of dust.
5.) Overclock your GPU (Not Recommended). I have a 5450 1GB, so it can be annoying when I get lag. I then used AMD overdrive to overclock my GPU, and I got less lag and better gameplay.
A Quick Note:
If I were you, I would get not 2GB GSkill RAM, but 4GB of Cheaper RAM. All RAM does exactly the same thing, so it really isn't worth it to splash out the extra cash. The MHz also does not really matter; as the latency will cancel it out.
It seems to be that the higher the MHz, the higher the latency. Here is a video you should totally check out: