I wouldn't recommend using XP64, no. 64-bit isn't quite the amazing thing some people think it can be (some think it'll double your speed, which it won't). The problem is that everything has to be built around a 64bit architecture, which is why you need a 64bit OS like XP64. Microsoft haven't done a great job on it though, and will be dropping it soon enough once Vista is out anyway with its own 64bit implementation.
Currently some of the GNU/Linux distributions like Ubuntu do a much better job of 64bit computing than XP does.
One of the main problems is that you need all your drivers and everything to be 64bit too, if you're going to get anything out of it. XP64 driver support is, as I understand it, not the best (certainly not like XP32). And I haven't seen that many apps being phased over to a 64bit architecture either.