Anyone who can't get Vista to run just a beautifully as XP doesn't need to be building their own computer.
Vista is a great operating system once it's tweaked... It's just not so great "out of the box" I'd compare it to a 1911. You spend too much on it, then it needs a good Fluff and buff, and it will last you forever with no problems :D
Haven't tried Windows 7 yet, but from what I understand it's "Vista Lite" They removed all the crap that was useless, and added a few new toys, making it faster. Every Vista machine I've touched has gotten the same treatment for free.
Get yourself a custom rig from Cyberpower PC or Ibuypower... As long as you can put up with waiting a few weeks for them to come in, they're pretty well built, and you can "usually" get them to fix it if it's under warranty... (Better than some Dell service centers) I've had friends buy from both of them and have been happy.
If you're good enough to build your own, Processor choice is just that, personal choice, I prefer AMD, but you may like Intel. Look at Number of cores, core speed, and cache.
A Quad core at 2.2ghz blows away a Dual core at 2.8
More L1 and L2 cache is a good thing.
If "money is no object" and you like AMD, go with AMD Phenom II X4 Processor 965 (3.4GHz), they can be found under $200 if you look hard enough.
If "money is no object" and you're an Intel fan... Well, the I7 is new, and pretty damn flashy, as well as expensive. Intel Core I7 975, you're looking at near $1000 just for the CPU:
http://www.tigerdirect.com/applications/searchtools/item-details.asp?EdpNo=4777383&Sku=I69-0975&SRCCODE=PWATCH&cm_mmc_o=2mHCjCVybgwczfgtCjCVqHCjCdwwpPersonally, My current machine is an AMD Phenom Quad Core 9600, with 4gb of DDR2 and a ATI Radeon 4670 (512 ddr4) and there hasn't been a game or program it won't run with moderate settings, and I expect that with a new video card next year, she'll last me another 2 years easilly. I spent less than $600 to build it about a year ago.