Maxtor is a great hard-drive brand. Nvidia is a pretty good video card brand, and an intel core duo 2 is a great CPU.
Get a DVD drive that can read CD's at 54x, DVD's at 8x at least, and write perhaps at 16x for CD's.
Avoid AMD and you'll be fine. Limit your spending (100 for each component will get you a mid range system).
Course now it's all Serial ATA and super magic connections so I don't know where we're up to. C2D CPU is the don. my C2D iMac runs on 85 watts and runs anything I throw at it. good temperature too.
Intel is way faster than AMD, brandon (?)
Get a core 2 duo 4300 or 6300. If you overclock it a little it's faster than current fastest CPU(at stock speed) for like $1000 cheaper
Core 2 duo is definitely the way to go.
For a graphics card, upgrade to at least a geforce 6800, a 7600gt is pretty good. 7100, 7200, 7300 are crap.
Try to get something with a 256bit memory interface thing.
I don't know much about ATI cards.
For the hard drive.. any 7200rpm drive will do I suppose.
For the optical drive a lite-on DVD burner is only about 20$(US).
Obviously you'll need a new motherboard for all these new parts.
If you're not a computer freak any new intel socket775 board should be ok.
Don't be conned into getting an MGP graphics port if you're buying an AGP card - most things say the two are compatible, but it's very limited in the cards that it can accept. You'd probably be going for a PCI Express one now, anyway.
I haven't actually ever heard of an MGP port apart from on my own motherboard, but it's something to look out for nevertheless.
Right now I'm downloading on BitTorrent, ripping a DVD in Handbrake (H.264/AAC 1009kbps), and encoding my recorded TV shows for iTunes (H.264 again) as it records a TV show too (MPEG2 stream, 8mbps).
2ghz C2D running at 75%. no actual slowdown whilst viewing websites, email, dashboard etc. can't find my temperature reading either. it's a very efficient, very powerful CPU. AMD had the upper hand till these beauties.
The Intel Core 2 Duos are very nice unfortunately they come at a cost. Until the Core2Duo range I'd have definitely advised you to get and AMD. The nVidia graphics card range has also overcome the ATi range - especially the SLI capabilities. Definitely go for PCIe on the graphics card slot and preferably get an SLI capable motherboard.