You need a good power supply, I'd suggest at least 600 watts since you're gonna have more stuff by the time you build. Get a mobo that has a pci express x16 slot.
the best easiest thing to tell if your so called 500 watt psu is good is to pick it up, i love when people complain when they need a new psu that is 20 watts less than the one they had so theyd rather go for the cheaper but more wattage that weighs less than their heatsink on their processor.
Sweet, I have one of the worst steppings there is for them (CABGE) and mine tops out at 2.8ghz. I have heard of people getting their 3700+ to 3ghz on stock voltage or a very little increase in voltage...
to actualy get the full effects of the card you will probably need to step up another gig in ram. comps are like the body. you might have a kickin heart but if ur veins arent so great they will backlog your heart and everything falls apart. . .well its not that serious on your comp but you get the idea.
i suggest the OCZ powerstream, i just got one and this thing is rock solid, it doesnt fluctuate at all when under load, also have at least a gig of ram and u might wana overclock your cpu a little, to 2.2GHz at least i think