Since you are building a new rig, go with an OEM version of Vista. You have to request it, as it will save you quite a bit from the standard boxed version off the shelf.
If that reduces the price enough, to a more affordable level do it. It will along with the 64 bit wersion will let you upgrade to the maximum ammout of ram the board and proccessor will support.
If you go with the 32 bit version you will be limited to the ammount of memory supported which is 3 gigs. Including your video card memory. So really, the 32 bit version puts a damper on the memory you can use, which can affect how games and apps run.
If you want, you can go here to maximize how Vista runs and slim it down so it works more like XP which makes it easier to get used to.
http://www.blackviper.com/
Don't worry much about driver issues. There aren't the issues there were a year ago with them, or the availability of them. And more show up all the time.
I wouldn't go any lower than Home Premium though. The OEM version shouldn't be too expensive either. I got mine last spring for 129.99 US, instead of almost 299.99 for the boxed version off the shelf.