If you're running virtual OSes concurrently, and demanding applications like compilers, memory might be a bottleneck for your workstation use; you should check to see if you're swapping frequently. If the motherboard and budget supports it, consider 8GB.
I'd worry more about the amount of memory than the speed, if you're swapping to disk. Getting a dramatic increase in RAM speed will likely require a new motherboard and CPU (I usually upgrade all three together).
For gaming, while the 9500GT is by no means a bad card, there are newer cards that can outperform it substantially; I'd think about an nVidia GTX460, which has 3 or 4 times the fillrate and memory bandwidth for under $200. You might need a new power supply to run it, though.
2you should first state what your actual problem is. – akira – 2011-04-04T11:41:41.213
3You need to tell us what you are using the computer for. We can't help you unless we know what types of problems you're having. Also, do tell us your motherboard. – AndrejaKo – 2011-04-04T12:04:56.187
True, sorry. The computer is used workstation and gaming. As workstation, virtual computers, multiple OSes, development. Gaming like Crysis 2, CoD series games. Current mainboard is ASUS M2N-E – Glendyr – 2011-04-04T13:03:55.977
Any new video card you put in that computer will be bottlenecked by the CPU first, then the memory. I don't think you'll be able to play Crysis 2 without some major (read: CPU & GPU) upgrades. To be honest though, you would be better off just building something new. – Breakthrough – 2011-08-19T00:17:13.957