That PSU has got to go. Its overrated and has poor efficiency.
Get a Corsair VX550 if you're on a budget, if you plan on doing SLI later, get the TX750. As for the card, no point in going ATI. Its an nForce mobo. You can go 275, but you say 300ish,
so, GTX 260 Core 216 + VX550 / TX750 with one 8600GT for PhysX