I am leaning towards the Q9550 build because I want high performance in games (will be using 3D Vision which needs at least 40FPS - better minimum of 60FPS - average in all games at highest settings at 1650x1200) and all good parts. I'm also considering an alternative i7 build by compromising on a few of the parts (graphics card, casing, and no third-party CPU cooler).
Price is similar but I wonder if it's worth it to go i7 just to get a stripped down PC. I can always buy a good cooler later on and overclock to the extreme, but for now I don't want to go over budget. The mainboard and RAM choices along with the processor were based on the overclockers uk bundle where they have overclocked this i7 920 to 4.0ghz
You shouldn't have any problem using Windows 7 RC on that hardware. I hope you will go get it before August 15th, as Microsoft is cutting off the downloads and new licenses for the Release Candidate at that time. The Windows 7 RC will work until June 1st of 2010, but it will start doing reboots every two hours starting on March 1st of 2010. Still that is over four months from the official retail release on October 22nd of this year.
The other thing to be careful of, is there is no "in-place" upgrade from the beta, RC, Pre-RTM builds, etc to the retail version. Be ready to do a fresh install when you get a retail or OEM version, make sure to keep your stuff backed up (save-games, settings, bookmarks, documents, etc). Invest in a large flash drive or an offboard drive for backups. You'll thank yourself in the long run. Keeping your stuff available even if your machine is down or rebuilt is worth it.
That said, go pick up the Windows 7 RC. It is good. I have it running on a six year old laptop and on a nice new Core2 Quad development box at home. It works really well and I'm rather pleased with it. It took a bit to get used to the new task bar ("super bar"), but it ends up being a lot faster really, especially with the Aero-Peek enabled.