W7 versions differ only in features, not in code - compare at http://www.microsoft.com/windows/windows-7/compare/default.aspx
we have a 6015A with an ATI 1950 (full height slot PXIe x16) and a TI dual FW controller (half height slot PCI-X) - IIRC 24 instruments were the limit on this type of computer because of the *only* dual channel 800 memory.
vista was troublesome on this machine but W7 runs nicely (use the latest catalyst drivers and if any W7 drivers are missing use the server 2008 versions)
christian
and remember: only a CRAY can run an endless loop in just three seconds.