All you would have to replace is the CPU, motherboard, and memory which should come to a grand total of $500 or less.
Now, I don't know how my system compares to yours, but in comparison to what I had before, it is significantly
faster. It feels like I upgraded to advanced alien technology or something!
My previous CPU, motherboard and memory was the E8400 @ 4.05GHz, the EP45-UD3P, and a 2 x 2GB 1200MHz kit of G.SKILL's "PI" series (5-5-5-15). At the time of my upgrade, I still had a GTX 470 and here's a summary of what I noticed: I think my E8400 was bottlenecking my 470 because my performance was noticeably better, and also because I got a major boost when I upgraded to the 580. So I think upgrading from that E8400 to the i5-2500K allowed me to see the 470's absolute fullest potential! I think I'm getting the fullest potential from my 580 too because DAMN it's fast.
Still: I don't know how much faster yours is in comparison to what I had before. I know it's faster than what I had, but I don't know how much