I've got a machine almost built running dual 5870s (was $100-150 cheaper than 1x 5970 for slightly more output).

Does the OS have any effect on the hash rates?  If Linux is faster, would running a GUI linux distro instead of command line have much/any effect?  I'm looking at either Windows 7 x64 or Ubuntu.  I have some spare keys for Windows 7, so the cost of the OS isn't a factor.

UPDATE/EDIT:  If it matters, I may add on a third and/or fourth GPU to the system later.  Running a 1k watt PSU in the rig right now with ample space and airflow.