Ok, I gotta ring in on this, just because I KNOW what the 4870 is capable of, I got one
My rig:
E6420 @ 3GHz, stock vcore cooled by the ZeroTherm BTF90 linked to earlier
ATI 4870 (was an XFX 8800GTS 320 G-80 two days ago)
XFX 650i Ultra
ThermalTake Toughpower 700w
2x 1Gb Corsair XMS Pro DDR2-800
That's pretty close to your setup, and the 4870 was a HUGE step up. I play Age of Conan, (and yeah, it's a bit buggy, the memory leak still hasn't been fixed, but get in a good guild and it's fun), and
can play max 'everything' with 8x AA and all view distances maxed @ 1920 x 1080 res and get 35~ fps actually
playing the game. High res shadows and water reflections from 'everything'. The card is simply amazing.
It also supports shader model 4, and DX-10.1.
I mean, any upgrade is a risk on how well the hardware will resell later, but the way this card is benching, wouldn't you think it'll be a while before it's outdated and in need of upgrade?
Only note, (and it appears you've got this covered, but it's worth mentioning), is that these new gen cards are getting too fast for many people's CPUs. We're back to processor bottlenecking. lol. IDK how well a stock quad will do when paired with this 4870.
Oh, and I think the single 4870 still beats those 88's in SLI. Probably more likely to beat them in 8x SLI.
There's no practicality in being a fanboy of either 'camp'.
EDIT: Given the two options of one ATI 4850 vs. two 8800GTS 320s SLI'd, it'd be a coin flip to me. Don't you only get like 1.2-1.3x the performance in full 16x SLI over one GPU?