When you say Ahtlon 3500+, this is the AMD64 3500+ right?
Oh the PSU it's from NZXT case, ahh yes, my Nemisis case had a 450W jobby in it. I blew the crap outta that in a week with my lowly 6600GT, (AMD64 3500+) Mine went up in smoke, black smoke.
Buy a new PSU quickly. In my sig see the pic of a good PSU for your rig. See the amps on the 12Volt rail? To run your 7800GT and the AMD64 with comfort you're going to need a 500W unit with 26Amps. Click on my BSODs link in my sig. Scroll to the PSU section, there's a link there to PSU specs. This gives you a complete list of all PSU on the market by spec. Find the one you like. Antec, Thermaltake, OZC are some of the best manufactures.
Cheers.