They did release a 64 bit XP, a few years ago, but XP support is going to continue only for so much longer. It might be dropped by the end of 2010 or so.
It's up to you of course, which to get first, but you're definitely better off having both a 64 bit operating system as well as a quad core processor.
Personally I'd recommend waiting for Windows 7 official release, and not buy into Vista now. Then again, you could get Vista now and simply upgrade later, might save a little money that way.
Or you could download the Windows 7 RC 1 from Microsoft and use it for free until March of 2010 when it expires. That's what I did, and I don't regret it for a minute. I loved Vista, but Windows 7 is so much better in so many ways.