4870 will still be the winner, because its raw power will compensate in most cases for the low ram count.
But limited to 8x PCIe lanes, you might bottleneck the cards to lower...I mean the 4850 is around the 8800 GTX power, and when I saw the review of 8800 GTX 8x vs 16x the tests can be identical or in some cases they can be a pretty big difference away.
in terms of raw power over all including ram count and future proofing it will be the 4870 512 over the 4850 1 gig.
IMO, the 1 gig of ram is wasted on the 4850, just like it is on the cards in that class. I mean its always good to have the extra ram, but the card has 256 bit.
Look at the 8800 GTX (ultra) for example, it has 768 megs which isn't really that much off, but it has 384 bit bus width, which aids in helping acheive more use of the more Vram.
I would say +1 for 4870 x's 2 any day over the 4850 since anyways its not that much more expensive now.
don't forget that the 4870 is 256 bit bus width 2, but its memory wall isn't there because it has the very fast GDDR5 to help it
@ 8x PCIe lane, ever thought of grabbing a 1 gig and waiting for prices to go down and purchasing a second, theres really no rush, a 1 gig 4870 pretty much owns every game, no need really to go all out
We need yoru resolution and your specs though to make a final judgment.
CPU: E8400 @ 4.05 GHZ
RAM: OCZ 1066 Gold 2x2GB
PSU: OCZ StealthXStream 600W
HDD: WD Caviar Black 1TB
running on Vista Ultimate/ XP Professional.
Cooling shouldnt be too big an issue, i've got an Antec three hundred with 5 120mm fans.
So the x8 lane width would limit the cards? Heard somewhere that when in crossfire its enough, but that could be BS.
Also, if i buy one 4870 now and one from a different manufacturer a bit down the line, what sort of compatibility issues could i expect? I've heard that the card BIOS can change between manufacturers so i'd have to flash one.
The 4850's would do slightly better but not much. Though with the price difference it may be the way to go. By the way... have you considered getting an x2 card????? You get the same performance but it'll operate in one x16 lane and leave a second lane for an additional card.