You will see some improvement, but probably not what you would expect out of $600 or so that you are putting into it. I would OC the CPU you have now as much as you can and get a good cooler for it, and get a 4gb kit of ram, then spend the rest of the money on a better GPU.
I have an original phenom 2.3, 3gig ddr2 800 ram, 8800gt.
Im building with 4gigs ddr3 1333, a phenom 2 720BE or an 810, a 9800gt.
I guess im wondering...will i even see a performance boost when i build this system?
Well I would do this...
4GB DDR2 + 720BE + 4870 1GB
Some money saved on DDR2 and save some more and get the 4870...
Else stick with DDR3 + 8800GT, save some cash and get something like ATI 4890 or Nvidia GTX275...