450W should be JUST enough as long as it is a nice brand name power supply.
EVGA is the top rated graphics card distributor. I have their GTS 512mb model which is a little better than the Gt, and I love it.
To save some money, get a P35 motherboard, but make sure you find one that is ready for the quad core without a bios update.. pretty sure most of them are, like the Asus P5K-E (100% sure it works out of box with quadcore).
You will only need 2gb of DDR2-800 memory for games, if you're feeling ambitious, you can get 2x2gb sticks of G-skill or corsair memory.