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.
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.