I don't think going with a Pentium D is a good idea, the Athlon 64 X2s are significantly better. In fact I wouldn't go with Intel for any desktop right now. If you want to upgrade in the future then going with an AGP card may not be the best idea either. For power supplies you should only go with recognised brands like Antec and Enermax.
If i was building a PC with that budget id probably get the following...
NForce 4 Socket 939 mobo with PCI-E - dont buy AGP now, you'll regret it in a year or two when none of the high-end cards come out on AGP.
Athlon 64 3200 Venice - supports SSE3, runs cool, usually overclocks nicely from 2GHz to around 2.5GHz on air. Wait for the Dual Core Athlon 64 X2's to come down in price, and your exisiting Socket 939 motherboard will be compatible with them after a simple BIOS flash.
1GB PC 3200 DDR RAM - get a decent brand like Crucial, Kingston, Corsair and you should be fine, and if you get 2 x 512MB sticks you can run them in Dual Channel mode which doubles the memory bandwidth.
As for the Graphics card, at the moment here in the UK you can pick up an X800 PRO for around £160 ($287) or the more powerful 6800GT for around £175 ($314), both of which are very good cards for the price-performance. If you cant quite afford one of those go for a 6600 GT which is around £105 ($188 ) and is still quite a respectable performer.
For the Hard Drive, I'd get a 160GB Hitachi Deskstar 7K250 SATA, plenty of storage, usually reliable, although you can get it in bigger capacities if you need more than 160GB. Dont bother with a Western Digital Raptor, they cost way too much, they are noisy, you get bugger all space (36GB or 74GB), the only advantage youll get is faster load times and boot up, although for the price, IMO its not worth it.
And, as others have said, dont underestimate the importance of the Power Supply you use, get a good brand name like Antec, Enermax, Hiper (Coolermaster) or Tagan. Dodgy £10 ones which claim to be 500 Watt are probably about as good as a 250 Watt Antec.
Anyway, enough rambling, I hope you find my opinion helpful.
Where in the UK can you get a X800Pro or 6800GT that cheap!? Anyway a X800XL should be cheaper than both of them and perform about as well as the 6800GT at stock (but it doesn't overclock very well).
If you're planning on overclocking your CPU, try and get RAM that's rated the correct speed so you don't have to run the RAM and CPU speeds out of sync. For example if you wanted to run a socket 939 Athlon 64 3200+ at 2.5Ghz you should get pc4000 RAM. I wouldn't concern yourself about running RAM in dual channel, the performance increase in very minor.
If you want a fast hard drive then I've heard the Maxtor hard drives with 16mb caches perform comparably to the Raptors but without the noise or low space (in fact they only come in large sizes).
Yeah, unless you have a complete surround system the A8N's onboard is fine. It has 8 channel support (whatever that is, I'm not friendly with sound stuff), and you can do some sort of surround sound with it. You might also want to consider the 7800GT at this point in time if you have the cash, it appears to be a good deal.
Building a computer is the best way you can go. Use Newegg to get really good computer parts/software for cheapie prices. It's really good. Both of my PCs are built from their stuff, and they both run great.
Fine Garbage since 2003.
-Paying off a massive amount of debt in college loans.
-Working in television.