Welcome to the forums, Tigger00!!
Is the PS you're using AMD approved? If not, you may have to get another one later. 275W sounds a little low for an XP, but if you're not running a lot of extra stuff (extra HDs or CD-ROMs) you might be able to get away with it.
As for your question, I haven't heard of a better PS lowering temperatures, so I'd say no.