I just downloaded a torrent of Vista 64 back when I used Vista because your 32 bit serial key works for 64 bit. Personally, I see purchasing software as purchasing the serial key, Direct2Drive and Steam just reaffirm that belief.
$10 to MS will get you a 64 bit disk.
Anyways, I use Ubuntu 8.04 as my main OS, however I also have it dualboot/Virtual Machine XP Pro. I never game on my computer anymore because my 360 has better graphics than my laptop could ever have.