what graphics card you going to be using??
As far as os check your devices if u can get 64 bit drivers then xp64 all the way.I wouldnt touch media centre with a 20 foot pole
If theres things you need to run in 32 bit then either dual boot xp64 and xp 32 or stick with xp32. I think xp64 is a cracking os mind far better than 32 bit for me
The reason I ask about gfx is if you already got an nvidia buy a sli board, Also make sure the sata 2 hdd will work on the sapphire board. Ebuyer has a 320 gig western digital sata 1 drive (wont be much slower than a sata 2 ) for 67 quid