The way I see it, always go with the biggest, baddest chip, RAM and video cards you can afford. There is no need to buy a $1,000 chip and a $750 video card. That's just dumb. Put the diamond-studded computer out of your mind and buy stuff you know to be a rung better than you would otherwise normally buy and be happy with it. If you scrimp to save a few bucks now, you'll not be happy with what you bought in a month or so. Always do yourself a little bit better than the bottom rung, and you'll be satisfied you didn't short yourself later on. Now, it is true that prices always fall on computer goodies, but that is the nature of the beast. You can't wait forever to buy the components you would like to install today. They become obsolete and newegg may no longer sell that item. You just take your chances the prices won't fall too badly and find you overpaid a ton for that chip or video card in six months or so. Good luck...
I'd buy the RAM for Vista. Vista supports 16Tb of RAM (in the 64-bit version, I think); XP supports only 3Gb. Problem is, Vista is a crappy OS with which to begin. I wish the Boys in Redmond would fix it. They could trash half the garbage in it and no one would notice. I might use it then...