Hi,
Well I would say faster = better, so go for a fast system, no Celeron or Duron, I would say at least an Athlon XP1800 or P4 1800, lots of RAM (512 MB - 1 GB), And a good soundcard is important as well, I don't know that much about soundcards, I have a SoundBlaster Live!,
And it works fine for me, Although there are better cards out there than that.
If you want to use soundfonts I would recommend an Audigy (2), There are also a number of softsamplers that can read Soundfonts, or you can convert them using apps like Awave.
Also I would recommend a fast and big HD, to store your samples and sounds (at least 80 GB, 7200 rpm ATA133)
Good Luck & Greetz
,
Marcel