I normally go with a bare bones and gear it up, but I do know what your talking about.
In order to put the latest and greatest video card in you will almost have to up grade the mother board. Memory may not and probably won't transfer from the old board. Hard drive depending on age can still be used. Power supply, unless you need it for dual vid cards, you can make that 450 work just fine, unless the amd processors eat up power. Unless you have to or have found something useful that you like, vista is pretty much junk. I would still perfer to use XP over anything else I have. You can still order most systems with XP also, which is a plus if you don't want Vista. I'm sure that eventually you will have to in either case, that makes me sad. When I did my last upgrade last year all I did was piggy back my old hard drive for a month and drag all the stuff off it I wanted. WoW transported great. Saved me having to reload that pig. I have a portable drive I keep it backed up on now. I don't know if any of my other games would have done that. I never installed much else but norton on this system to keep it clean of excess processes for WoW gaming. I'm almost thinking that I will go with the AMD processor then next upgrade cause I don't hear much difference is performance between the two anymore and the price would allow me to get a little nicer system.
Now depending upon age of the old board, standards have changed a bit in connectivity of the hardward and you may not be able to use any of the current parts. Alot of ribbon cables have gone to a much smaller connection along with the power plugs. That's why I have gone with bare bones systems, keeps everyone happy that way.
Anyway, I tried to keep to the original question and hopefully you find enough help from this and other posts.