Good quick comparison on MS's site here: http://windows.microsoft.com/en-us/w...oducts/compare
I honestly can't recommend Ultimate, even if it is only $20 more.
For the components, I would recommend changing to the i7-920 (socket 1366) since it supports triple channel RAM. I think the benefit you get from the triple channel will probably be more than the benefit you would get from the extra 140MHz clock. I'd also go with the 5870 over the GTX295 if you can find one. They're cheaper, perform about the same, and have DX11 support.