Do what I did, and try it out first before commiting to it.
I set up a new system with WinXP x64, and installed all the critical hardware and software in it and made sure it worked (and made some changes as a result). But I didn't overwrite my existing 32-bit XP setup, so that was still available in the event that x64 didn't work out.
Perhaps the easiest way to do this, if you're reluctant to buy a whole new computer, is to get an additional hard drive for your main DAW, and install x64 on that.
There is a 120-day free trial of WinXP x64 that you can download from Microsoft, which is what I used.
Note that, unless they've updated the trial, you'll also want to install x64 Service Pack 2 (which is not the same as XP SP2, just to confuse you).
If you think you're going to be installing the trial version of x64 more than once, you might consider combining SP2 with it on one installation CD.
Note that, if you do decide to buy x64, you can't buy it as a retail package, but only as an OEM product, which means you might have to buy some random piece of hardware with it.