In the past, this debate usually took the form of better gui vs. efficiency. Windows Vista does look better and comes with a few extra features, but early on it typically performed worse than Windows XP. Tom's Hardware did an excellent benchmark
here.
However, after about a YEAR AND A HALF, the performance of Vista has been improved (mostly by improving drivers) to about the same as XP (sometimes slightly better). If you can stand the ads on the page, extremetech did a benchmark of Vista sp1 vs. XP sp3
here.
As a sidenote, dx10 is available for Windows XP. GET YOUR MOUSE OFF THE REPLY BUTTON and let me explain

Originally, the purpose behind restricting dx10 to Windows Vista was to force a new standard of GPU memory virtualisation that can't be done in XP. However, Nvidia had problems crafting drivers (ATI did it just fine) so Microsoft decided to make this requirement optional, removing the only barrier keeping dx10 from working on Windows XP. I'm not sure if I can post it on these forums, but the curious can search "dx10 on XP" and come up with some interesting results to learn more.
Overall, the only reasons to stay with XP are if you really like it and don't want to try anything new, really enjoy old apps not compatible with Vista, or run an older computer where every bit of system resources counts.
If you want a 64-bit OS the choice is even more clear, as Windows XP-64 bit is buggy and poorly supported. MS put it on the same development cycle as Server 2003 and has not indicated further development, leaving it with only sp2. 64-bit computing is about more than access to a larger pool of RAM, and I recommend reading an excellent article on the subject
here.
If you had written your post a year ago, you'd be receiving much different advice. However, as things stand now, Vista-64 bit is the way to go (premium or higher).
*I cannibalized my reply to basically the same question from someone else for this thread, editing it as needed.