Thank god you are going with AMD. They build the best high-end budget pc's. As long as you aren't working for a gaming company, there is no reason why you should waste the extra money.
If you are looking at gaming, you can "honestly" go with dual core. Games don't utilize the quad core technology just yet. Even a dual core 3ghz processor can run crysis on max specs. The thing which holds you back is the video card.
As for your list, you can definitely find a cheaper power supply on newegg. Since you probably won't be overclocking your pc, (really no need), I would suggest finding a good 500 watt ps. Here is a good 580 for under 20$. You only need 700 watts if you are overclocking or running sli cards, (which you aren't)
http://www.newegg.com/Product/Product.a ... 6817709011I also suggest getting at least 3gb of memory, especially if you are running vista. It will help on load times and hammer. Try finding memory with a heat spreader. It will increase the efficiency of your memory. If you are trying to build a gaming pc the difference between ddr2 800 and ddr3 1066 is really negligible.
Lastly, I am more of a geforce guy, (the 260 is a good budget gaming card), however I honestly never used the 4850. It is probably a fine card.
All and all, not a bad build. I have a lot of experience building budget gaming pc's for people. I don't mean to knock whoever built it, but if you focus on exactly what you want you could probably shave 50-100$ off of that price.