Hi,
1) Every game varies in how much CPU vs GPU processing it uses on average.
2) With some EXPERIENCE we do know that you have a big bottleneck in most games so you would get over 2X the FPS easily when paired with a modern i5/i7 CPU.
You can use Passmark as a rough guide:
http://www.cpubenchmark.net/cpu.php?cpu=Intel+Core2+Duo+E6750+%40+2.66GHz
About 1000 score per thread. That's meaningless to most people, but when you look at a lot of benchmarks it starts to make sense. It's also useful to compare to any CPU you might buy. At minimum you want a dual-core with much higher score, or at least a 4-core with the same.
3) LGA775 CPU's?
http://pcpartpicker.com/products/cpu/#k=13
USED on E-bay may be the way to go, but I don't have time to look. Here's one, the Core 2 Quad Q9450:
http://www.cpubenchmark.net/cpu.php?cpu=Intel+Core2+Quad+Q9450+%40+2.66GHz
It's a little bit better per core, and has four cores (vs two). So you'd do better but again it's going to VARY a lot depending on the game. Probably between 10% and 60% better depending on how well threaded the game code is but that's an educated guess.
4) Make sure motherboard supports it
5) Overclock if possible
Summary:
I hope that gives you a rough indication of what to expect. You won't do miracles on such an old platform but you can do better.
Other:
If you have 32-bit Windows then you really need Windows 64-bit. Since you have a 2GB video card, with 32-bit Windows you'd only have a max of 1.8GB memory roughly available regardless of how much system memory is installed.
Other:
There are some games that would run okay with your setup. You might want to investigate that as well. And for the record, I nearly didn't even reply since using foul language annoys me.