HTPC software? OS?

[FlyingFish]

Ive just built up a htpc for downstairs And have a few questions to help improve the experience.

Following specs:

Asus P5QL/EPU mobo
Intel E7200 cpu
ATI 3870HD
500Gb Samsung F2 5400rpm quiet HDD
2Gb OCZ memory
Wifi card
Windows 7

I have everything running and installed, and have been playing around with Windows Media Center, and just now XBMC. I would like to get straight to the action as fast and as simply as possible

I was wondering what programs people are using for their HTPC's or even OS's.
Something fast and easy but also flexible too.

Already I've gone off Windows Media Center slightly as K-Lite media player plays nearly everything visually better, but its a bit old school and requires the windows OS.

I like XBMC, and have tried the XBMC Live from a usb stick. Quick and handy, but probably not making use of the hardware, and certain parts are a bit screwy with my wireless keyboard touchpad.

Also would it be a quicker system (boot up/operation etc) with Windows XP rather than Win7?

Cheers
Mick