When choosing a psu, you can't just rely on the rated wattage. A psu is one of those items that you can't skimp on. It touches every part in the system. If it blows, it can take something with it. The better manufacturers are corsair, pc power & cooling, antec, OCZ, and seasonic. I suspect a 'mild' gaming rig will require ~500w from any of the above manufacturers.
Well, if you are only going to use one video card and don't plan on using mutiple video cards, then a 750 watt power supply is more than sufficient for your needs. The very high quality psu's such as the Corsair TX750 you are looking at is designed for high end systems with two video cards in SLI or CrossFire mode.
If you are positive that you will only be using one video card, then a high quallty 500 watt power supply such as the Antec Earthwatts EA-500 will meet your needs. It can power any single video card made.
Here's a link to a technical review of the Antec EA-500:
1) the Q8200 only has 4mb of cache and a 2.33ghz clock speed while the P2 940 has 6mb of cache which the architecture handles far better and has a 3.0ghz clock speed.
2) 4770's when oc'd are better than the 9800GTX+ and use about 30w less, also cost less aswell. they get to about 850mhz if you use the rivatuner hack and even further if you have the ASUS TOP edition and up the voltage although they can get very hot.
3) you need a amd socket motherboard for an amd cpu and nvidia chipsets are rubbish.