I'd like to add some personal experience in here in the driver department.
I built a new computer and I put Vista 64 bit on it. I came to find out that my PCI Sound Card does not have 64 bit drivers for XP or Vista, and as far as Vista goes it only has 32 bit drivers. I ended up putting Vista 64 on it without doing a lot of prior research. Everything is supported except my sound card.
Fortunately, there's hope for me. I have an onboard sound card to use for the time being. And on my sound card's web site, they have a specific notation that in Dec of 2008 they expect to have Vista 64 drivers available. That's 1 month from now. So, not a big deal... but I'm encouraging EVERYBODY who is considering on going to 64 bit Vista... please... track down every driver you would need and make sure the 64 bit drivers exist. Most new hardware will support 64 bit, but don't just assume like I did. I thought because my computer was brand new I was okay. And come to find out, I'm 1 month short of being "aye-okay"