High 40's for idle is a little on the high side, but not so much so that the CPU would throttling back that much as to make the games run that slow... And the temps definatly don't justify the system freezing. I think there is something else going on there.
I would definatly upgrade that PSU, while it may or may not be part of the problem, upgrading it will definatly prevent future problems from occuring. My parents PC had a generic 350 watter on a system far less power hungry than your own and it reciently went out on them. So I gave them my Enermax 350 and got an Enermax 550 for myself
One thing i'd do if you haven't already is to make sure you have the latest video, chipset, and sound card drivers. Also make sure you have the latest patches for whatever games you're playing.