I'm afraid I'd also recommend going with windows 7. Vista really was the O/S that should never have been allowed out of QA.
I've used Vista professionally, since Microsoft released it, and eventually couldn't wait to ditch it. When installed, it was reasonable - but gradually became slower and slower, and more erratic. Their fixes didn't really improve it. Having free access to Win7, I bought another couple of hard disks - and after testing it, have now fully migrated over. On the same hardware, doing the same tasks, it's faster and more stable. The only compatibility issues are minor annoyances - such as the inability to use unsigned 64bit drivers without either hacking things, or doing a convoluted F8 boot. As long as the drivers are signed, the Vista one is usually accepted without a problem (and I've never found a driver signing issue with Win7 32bit anyway).
As far as PSU's are concerned, 550w should eat almost anything that you care to throw at it. As a previous commenter has said, it's probably better for reliability to run a big PSU under limited load, than a smaller one maxed-out.
My own experience is that I expect a windows machine to typically last 12-24 months, before it needs flattening and rebuilding. Go for as much memory as you sensibly can. Windows can't manage virtual memory to save it's life - so don't let it go near its pagefile. Give it plenty of RAM instead. And finally, always partition the disk so that your data is kept seperate from the OS and apps. Makes it easier in the long run when Windows does let-go, and you have to reach for the OS disk.
S.O.