I also realize that running any Dx 11 capable Gfx cards on Vista SP1 limits the card to run only on Dx 10 and below, i've been using SP1 ever since cause i dont really like what SP2 offers and there have been issues after i upgraded to it the very first time. So its kinda a dilemna for me now to either put up with the issues and get Vista SP2 or just keep with it and not use Dx 11 which pretty much defeats the purpose of getting the new card in the first place...
On Vista SP1 you have only DirectX 10.1 support, though not all features are present. I can't find a reason not to upgrade to SP2 if you're using Vista. The benefits include Bluetooth 2.1 support, better WiFi support, improved HD video playback, better power management, integrated support for BluRay recording, and a few others.
DirectX11 is not present in SP2, you'll need the Platform Update for that, which includes a few updates regarding XPS support and the Windows Imaging Component, support for MTP over Bluetooth, and, of course, an updated graphics runtime consisting of: Direct3D 11, Direct2D, DirectWrite, DXGI 1.1, and WARP.
Unfortunately, the Desktop Windows Manager still uses only DirectX10 features and GDI/GDI+ will still use software rendering.