You didn't mention what software you are or are intending to use. If it either of the 'four' popular sequencers, then first things first: the RAM- get as much as you can because this decides the speed, number of tracks available and reliability of your system. Next, try to get a Zip drive for storage of songs/samples or better yet, get a CDR-W writer.
With regards to synths (if you intend to use them), you can opt for either the software equivalent (cheaper, more flexible) or their hardware equivalents (real thing which collects dust). Get either a EMU, Korg, Roland or Yamaha.
You'll also need a good AKG C1000s for recording vocals (perhaps) and a dependable Mackie or Soundcraft Mixer with at least 4 channels. Oh, and don't forget a pair of quality-branded speakers for mixing. That's it. As you can see, there are loads more to discuss about but the list may be too long!
Hope this helps.

WINN