If you are building a new system, 65 bit is the way to go. Anything that has the "Vista Ready" logo, which would include any kind of peripheral such as keyboard, mice, printer, etc. To get that certification, the items were required to have 64 bit compatibility, which essentially weeded out almost anything on the hardware end that had a problem with drivers and such. There are still some obscure software programs that don't play nice, but in most cases, the 64 bit operating system does just fine with them.
Since Windows 7 comes out in 2 days, you might as well go with it. If you do buy vista, you should get a coupon for a free upgrade to 7. Depending on who you believe, the backlash we saw when Vista came out of people fed up and going back to XP, isn't expected. Win 7, in appearance and user interface, is pretty similar to Vista, so if you aren't a tweaker, and don't often climb into the bowels of your OS to play with settings, you'll be fine with 7.
Good luck with your first build!