I'd just go for 64bit version. I don't quite see the point of going with 32bit then paying extra for 64 then the hassle of upgrading it, I mean I know it's only $10 or whatever it is in the States but I've had no problems with 64bit and I know my hardware aint exactly cutting edge stuff. My ATI graphics card is an X1850 which is DX9 and I only have 2gb ram and 3 5200rpm ancient WD harddrives and OEM 64bit Vista HP works a treat! If you're doing a new build unless you're buying some very specialised hardware I wouldn't have thought you'd run into any great difficulty with 64bit HP.
If you run any 16bit software then 64bit won't like it, you might have to run a virtual machine or something to get it to work. Also all drivers have to be signed, tho that said I haven't run into anything which has caused much of an issue. I've had fewer problems with 64bit vista HP than any other Microsoft OS ever, and I've used them since Windows 3.1. Like anything new you'll find a few issues but it's not rocket science to overcome them. Just a bit of searching on the internet will find most of the answers you'll need.