SLI OR CROSSFIRE: Not sure- leaning towards crossfire b/c of DX11
Just FYI, nvidia's new high-end GPU ("Fermi"), the first nvidia GPU to support DX11, is currently scheduled to begin production in or around April of 2010. If that was the only reason you were leaning towards ATI GPUs, it may be worth waiting to see if they push out other cards along with Fermi to compete with the rest of the ATI 5xxx series.
Accourding to the tests done by this site 2 5770's accually outperform 1 5850 in some cases, and they are less expensive.
He mentioned a single 5870 not a 5850 !! - and the 5870 will perform better but is about $50 more - but if you drop the one 4gb set of RAM you still have a $50 net cost decrease and get better performance and should you need even more video performance at some point in the future you can add a second 5870 when prices drop !
4GB is still plenty for most people (unless doing heavy CAD or video editing) most games will not use more than 2GB even if there is more available. Since most users are still on a 32 bit OS games are designed to run in the 32 bit limits of a total of 4GB of addressable RAM.