The thing is that this rebate is from a reputable company(not some rebate center but from EVGA directly) and as long as you mail it in with the correct info on time, it's almost guaranteed you'll get your money back.
I'd say conservatively, the 8800gs is 30% faster than the X1950Pro and it's probably closer to 40% on average. Also it supports DirectX 10.
I have an X1950Pro(well a X1950GT at pro speeds so it's identical). I use it in my main computer. It's an excellent directX9 card but it is showing it's age. I'm running it on a AMD Athlon 64 3400+ so it's pretty close to your setup, in fact somewhat faster and tell you the truth, it's not all that fast anymore.
Don't get me wrong, it's good but not quite as good as it could be. I game at 1280x1024 and though games run, it's more of a medium thing.
I ran Crysis at low 1280x1024, no AA mid 20s FPS.
Test Drive Unlimited ran at High but with no AA and no HDR, I was just playing it and I usually get ~26fps which isnt too bad for a racing game.
Call of Duty 4 I think I ran at medium-high with 2xAA, i dont remember the specific fps but it was okay.
I ran Warhammer 40K at all high but it can chug in larger firefights (high teens)
Oblivion Medium High, bloom, no HDR, no AA and outdoors it slows down quite a bit (again, in the teens)
I'll be installing Company Of Heroes, Bioshock and some other games soon, but overall, performance wise, it's starting to show it's age.
One good thing I can say about it is temperatures though. With the stock cooler, (Sapphire X1950GT, overclocked, 5% overvolted) it never ran above 42C idle and something like 52C load. I put on a Thermaltake Duorb recently and it hasn't gone above 38C idle and 46C load.