Building gaming+music production rig - questions

Hello, I am putting together a new rig.

PURPOSE: a fair amount of gaming, (such as bf2142, crysis, aa, hl2, etc) - achieving smooth gameplay on medium to high settings. A good deal of music production such as FLstudio, Guitar tracks 3 etc, home recording. Also a bit of graphic design via PS and some other programs.

BUDGET: anywhere under 1000$ - considering I already have a sound card, graphics card, and power supply.

I already have a Dynex 400 WT PSU... a SB X-Fi Fatality sound card... and an ATI X1600 pro 512mb from my old rig, I plan to use all of these in my new rig or at least the sound card, not sure if the video card will do what I want on some games, but its done fairly well so far... and not sure if the PSU will hold up for the parts I plan to put together.

This is what I'm currently considering:

CPU - AMD Athlon 64 X2 6000 AM2
Mobo - Not sure... don't know much about motherboards, presumably an AMD board? Any recommendations?
RAM - 4GB DDR2 6400 (Recommendations on brand? Kingston? Corsair?)
HD - 500GB 7200rpm
DVDRW - optical drive
CASE - Any
OS - Probably Vista Home

Question 1. I'm debating what OS to go with. I know a lot of people dislike Vista but my previous PC used Vista Home and I was one of those people who had absolutely no problem with any games I played and I tried like 20 games on it. so I might just go with Vista again. Am I right to assume that the 64 bit version is a waste right now even tho it expands the RAM capacity? I'm fine with my 4GB acting as 3.3 or whatever... for now at least

Question 2. Do you think my 400 WT PSU will be good enough for these parts, and even allow me to possibly upgrade in the future say my graphic card? Or would you recommend something higher maybe 500 or 600 wt?

Question 3. Any recommendations on perhaps swapping my x1600 pro with something more high end for my gaming? I know its more of a mid ranged card...