1. Some of those prices are weird. Really, I've seen 1.5TB for $125, yet on that thing there's a 1TB drive for $150...
2. Core 2 Duo is an useless thing to buy these days. Get a Quad or i7 for a new computer. Look at the best price-performance ratio, such as the i7 720. It is possible for you to try AMD (cheaper than Intel if you like to overclock), but it seems you're set on Intel.
3. Rather than a single HD 4850, I'd suggest TWO of them, or a single HD 4890. You could also go with nVidia, in which case a GTX 285 would be the best (and most expensive, except for the GTX 295) choice. GTX 275 seems to be about even with the HD 4890...
4. 4GB of ram, and 64 bit XP (cheap, and you can wait until W7 comes out). Unfortunately W7 is going to be RIDICULOUSLY expensive - starting at $250... the 64 bit XP you could probably get OEM to halve the price...
5. Buy as much as possible from retailers if you can. If you have a good retailer near you, such as Frys Electronics, buy from them rather than online. You may get the full sales tax, but shipping costs are eliminated (for all of the computer parts combined, the costs can be anywhere from $40 to $200) and you get the parts instantly. Sometimes you can nail some sales and get parts for VERY cheap (I got 4GB of 4-4-4-12 latency DDR2 800mhz RAM for only $45).
6. The case is important. Very important. It doesn't have to be a refrigerator, but it MUST be large enough and must have good air flow. DO NOT buy a flimsy piece of crap, even if it has good air flow, because it will not last in the unideal environment a desk or home floor can be. That $60 for a PSU AND case, AND mouse, AND keyboard, will give you utter crap. You should be spending $60, at least (unless you get a deal), on the case alone.
7. Cheap PSUs (Power Supplies) WILL **** YOU OVER. It doesn't have to be modular, but make sure it has enough power and MOST IMPORTANTLY MAKE SURE IT IS RELIABLE! If the power supply fails, it can either be: a. an annoyance - you have to replace it. b. a problem - it shorted out your motherboard and CPU costing you a couple hundred. c. a massive ****ing emergency - it started smoking and burnt part of your case and shorted out most of the parts in your computer, rendering most of your equipment destroyed. You don't want to take the time to sue the PSU company (chances are you'll lose anyway) - you'll simply lose that $1,000.