what your best off doing is checking the system requirements of games your likely to play. this will give you an idea of just how much muscle your system will need.
you didnt mention what type of processor you have ( pentium D doesnt suffice ) need to know 64 bit , 32 bit etc etc clock speed, single core dual core...
graphics card is also somthing your going to have to think about, but again we would need to know if its AGP PCI-E etc
if your thinking of vista, then youl want another GB of memory, and a bigger hard drive