Im all for tweaking and customising things.. I would love to get one of those new 8800 cards and put it in my 2.4ghz 20" iMac.
Im not so sure the alu 20" iMacs have soldered gpu's though? I only say this because in the iMac freezing fiasco a few months back, some folks got their gpu's replaced under warranty. 20" & 24". The last of the white 20" iMacs however, were definitely soldered.
I realise it would not be a 5 minute job to swap cards however.. Still, I like a challenge 😛 But yes, I doubt it would be easy to get hold of the 8800 gfx card in the first place. And even if you could, I expect it would be expensive.
Having said all that.. The Ati 2600 in my alu iMac performs pretty decently (I play UT3 in Winblows/Bootcamp). All settings maxed, bar the percentage thing which is at 60 or more %. Get between 30-45 fps for the most part (25fps at worst). The problem is, the damn thing still freezes occasionally, and it drives me mad 😡