OK I'm looking at two new computers to buy, I'm torn between the two and the final deciding factor is which RAM card is better,

One has a 3GB DDR3
The other has a 8GB DDR2
I'm looking for the one which would be better for playing games on

8GB would be better. I have 6GB DDR2 RAM installed on my computer and the gaming is superb

Note: 32 bit windows CANNOT use anymore than 4 Gig TOTAL memory, so the 8 Gig setup would be running 64 bit windows..

64 bit windows, would that be a problem?

Not a big problem. What is your current set up? It's easier to help when knowing that. Some key factors are

Operating system, age of computer, Hard drive, processor.

I'd definitely opt for the bigger RAM. But as was mentioned earlier, make sure you are OK with 64-bit win.