OS: Yes W7 is the recommended Windows OS.
Best config ? Scratch is GPU based, so go for a good GFX card (ATI Firepro, nVidia high end QuadroFX 6000 or latest K5000). If you don't need SDI out and $$$ is a limitation, then get a good GTX 680.
The rest will be disk access in R/W to ensure real time play back.