Just upgraded the machine into a modern gaming rig. Should be miles ahead of what that 8800GT will do in games.
Also, just saw that you are running Kingston HyperX RAM on that 2500k. Kingston RAM runs at 1.65v, which is dangerous for modern Intel processors, they weren't designed to handle more than 1.5v. Unfortunately this means the warranty on the processor is now void.
I would swap out that kit for some sticks that run on 1.5v or less, so hopefully there wont be any added damage to the processor later. Most kits from Corsair or G.Skill run at 1.5v.
The GTX660Ti would be a very beefy upgrade compared to your current GPU...However, you may want to consider getting a lower wattage PSU, such as a 550w. You won't need more than that even if you end up OC'ing.
On a side note, have you considered the Radeon HD7950? AMD announced a $50 price cut on these cards so they're a very formidable option considering the performance they're capable of.
They are about 50 dollars less, but surely performance can't be that far off. Perhaps take the money saved, pick up a nice SSD. I recently got a 64gb ssd for my older rig.
Myself, still soldiering on with my Athlon II quad core, AM2 board, 6gb of ddr2 and GTS 450(rig does everything I need, what can I say? Next upgrade probably a better PSU and video card though).
But, I have a 1 tb mechanical drive that I use for storage now. I added the 64gb that I picked up for 50 bucks at Microcenter, and now Windows 7 is a 7.0 overall. I think I'm rated at 7.3 on my processor and 7.2 on ram and graphics. My SSD is supposed to be SATA 2, so I think my older board is holding me back. But even then, performance is great. Applications are snappy to launch, there is literally almost no welcome screen as I wait for Windows 7 to boot. The biggest delays in boot time for me are waiting for my PC to post and getting to the Starting Windows splash screen.
I see you quoted me while I was editing. No question that the other cards are better, but from an 8800gt, a 570 would be a MASSIVE jump. As I said, I've still got my GTS 450 running, and I noticed a good improvement from the 9600gt I used to have.