AMD Athlon 64 X2 4200+ CPU
5 internal hard drives,
Not a chance that'll work. The watt ratings on PSUs aren't all that meaningful, but a nominal "450W" is really low end. (What you actually need to look at is the current drain of each component, and the maximum supply on each rail - there'll be a sticker on the PSU listing it. Usually the video card and hard drives are what get you)
Is my Win7 drive somehow dependent on the WinXP drive?
I would go for a minimum of 550watt for yours but would strongly recommend a 600-650watt. It is always better to have more than enough, than to run right on the edge. Plus there are so many factors involved. The TRUE continuous ability to supply power, what peaks it will handle, how stable is is. This is the reason a good power supply cost more than a cheap one with the same rating. PSU's can cause a multitude of problems. You may well indeed be under supplying the system. Besides it NEVER hurts to have a good PSU.