FRAPS is the best real world in game test. 3DMark06 is saddled by the CPU test (what is the purpose of that?) that drags down many a score. One thing about most benchmarks at Tom's, Anandtech etc. is they generally are done with very high end CPU's, whereas many gamers have mainstream CPU's that sometimes limit their benchmarks. I know mine does.
You'll see a world of difference. An 8600gt isn't all that much better than a 7600gs and an 8800gts 512 is a good card. I went from a 7600gs to a 3870x2, and it was amazing, even in games that didn't completely support Crossfire.
Still, if you can wait. G280 will be out soon, as will the 4xxx series. This summer and fall will be a great time to buy GPU's, and buying last year's technology so close to new cards might be a letdown.
I bought my 3870x2 five months before 4870 will arrive with similar power and no Crossfire scaling issues, and yet I feel that I bought too soon. I generally like to keep a card for at least a year before I upgrade.
Do you use Vista or XP? A friend's 8600gt tanks out in LOTR Online DX10 mode (his game freezes) every once in awhile and he's had to restart his computer when we're in the middle of a fellowship. Yes, the tank's Dell tanks out while the squishy minstrel is left to handle the aggro. LOL.
So I'm glad my wife's joined us in game now so I have someone to rely upon. I know not to buy $2000 Dell's with 8600gt's, so the PC's I build can generally handle the game. At any rate, an 8800gts 512 will handle DX10 without tanking out.
8800 GTS is still a great performing card, and will be for a while, no games i have thrown at it slows it down, i mean none, granted i dont have crysis, but i hate that game, anyhow.... Seeing as how you said it will be arriving next week im going to assume you already bought it, dont worry, your going to be quite happy with your purchase now, and well into next year.