The nForce is just nvidia's name for the motherboard chipset. I am not aware of any issues with using an nForce mobo with an ATI card. If you want a cheaper nforce 2 mobo, I have heard that the Epox 8rda is a very good mobo.
Other than that you really want a good quality power supply like
the Antec True Power.
This link should help you to pick a good powersupply, and they also give good prices on power supplies as well.
http://www.directron.com/psu.html