I have a Sapphire video card with ATI chipset in my new computer, it's totally supported in W7. All I can think of is that you may have an incompatibility with the Adaptec's firmware, which wouldn't be detected by the compatibility check (it actually only tests installed software – programs and drivers). There's also a known XP/Vista limitation with RAID arrays: the system partition may not exceeed 2TB. I have no documentation about whether this limitation also applies to W7. I also don't know for sure whether this limitation concerns the partition that contains the boot files, or the one that contains the OS files, or both, in case they're different partitions.
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"Earth is the cradle of mankind. Nobody can live in the cradle forever" (Constantin Tsiolkovski)

