It's sad that Toms put out such blatantly horrible charts and people are referencing them. Go to some other sites for valid GPU charts. Tom's really dropped the ball on their latest charts using a slow CPU and outdated drivers.
I have 2 identical systems one with the 4850 crossfire and one with 4870 1gb. from looking at most reviews the dual 4850 are better. and my 3dmark06 is higher as well (look at my system configuration) but i can't tell the difference in game.
What size monitor do you plan on using?
Except @ 2560x1600 resolution the 1GB 4870 doesnt make that much difference that you cant use the 4870/512 and 4850CF benchmarks to make a fair guess at the 4870/1024 vs 4850 CF performance.