Windows 7 will get support until 2019, so it's going to be the next XP. It'll be around for a LONG time. Besides, all the underlying features of Windows 8 like DX11.1, new file system, etc., will come in the form of a Windows Update to Windows 7. It happened before. Microsoft issued a Platform Update to Vista to bring DX11 and other big features of Windows 7 to Vista. If you buy Win7 now, and really want Win8 when it comes out, the upgrade to Win8 Pro is $40.
Also, TH did a test on the Developer Preview of Win with the FX 8150, and it did improve somewhat, but don't expect it to beat the i7 or i5.