As for now, Vista does not fully support OpenGL. And OpenGL is one of the things you DEFINITELY want to have when using 3D graphics programs.

Some antivirus programs don't play nice with XP Pro 64 bit, since they're developed for the WinXP 32 bit code base, not for the 2003 Server code base (which is what XP Pro 64 bit is derived from). Sounds bad. On the other hand, XP Pro 64 bit is much less of a target for viruses, trojans and worms, those nasties focus on XP 32 bit. So if you don't run p2p software, if you don't allow your kids to click on just about every rancid URL out there on the Internet, and as long as you have a decent (hardware based) firewall between your machine and the Web, you shouldn't have too much trouble.

Another problem with XP Pro 64 bit is lack of drivers. ATI may have corrected the issue, but for a very long time they either didn't have 64 bit drivers at all, or very crappy betas. nVidia has always had good solid 64 bit drivers, for both chipsets and graphics cards.
Then again, you wouldn't want an ATI graphics card anyway. The hardware is fine, but the OpenGL support is traditionally crappy with ATI. If you want OpenGL, you want nVidia.

Applications that don't delve as deep into the OS as antivirus software does runs just fine on XP Pro 64 bit. I'm running 64 bit XP on one of my workstations, and everything just plain works. Fast and reliable.

I do not know for certain whether Intel provides good XP Pro 64 bit drivers. Intel does have a reputation to uphold, however, so I expect they do. Nevertheless, if you're going for an Intel setup, check the driver support before you go for it.

One thing is for certain: if you choose a mainboard with an nForce chipset, you will have good XP Pro 64 bit drivers. 

I don't know if you have certain requirements for sound. For me personally, sound is not a major factor in computers, the onboard AC97 sound systems are sufficient for what I do. 
Many mainboards use a Realtek AC97 sound system, and good XP Pro 64 bit drivers exist. One of my machines has a Creative Live 24 bit system onboard, as part of the nVidia chipset, so it also had a good 64 bit driver.

The only piece of equipment that I haven't found a final XP Pro 64 bit driver for is my printer. The beta driver provided by HP works fine, by the way, so I'm not complaining.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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