4870 is twice as powerful. dont even compare them.
the 8000 series is old. nvidia has had the 9000 series since then, and they are now on the 200 series which includes the gtx 280 their flagship graphics processor.
the 4870 is slightly less powerful, but it handles higher quality, and higher resolutions with AA and AF quite well. the two cards are very comparable, but i went with the 4850 since it will run almost any game on high settings (1280x1024, maximum for my monitor).'
oh, make sure you have a good power supply, i dont know how much amperage that power supply has but you'll need plenty if you get dual 4870's or a 4870x2.
