Starkeeper if you are going to just run average quality soundfonts which sound as good as a top of the line keyboard then I would make the following recommendations:

CPU speed? 1 Ghz (could go somewhat less)
RAM? 512mb to 1Gb - to load soundfonts into memory
Soft synth? - LiveSynth Pro (works best with Yamaha styles), VSampler 3.0, RGC sfz (or +) http://www.rgcaudio.com/sfz.htm
Where would I find those soundfonts? http://www22.brinkster.com/guraydere/ http://www.sf2midi.com/ http://www.personalcopy.com/home.htm http://www.hammersound.net/ http://sf2.chez.tiscali.fr/soundfonts.htm

There are many more. You could do a search on Google.