get the 6870, 100 bucks faster than the 5870, equal in most respects, slightly faster in others.
check out passmark.com's benchmarks right here
http://www.videocardbenchmark.net/high_end_gpus.html
then look at prices.
AND LOOK OUT! the 6870 didnt fit in my computer so i took a hacksaw to it
damn harddrive rig.