I came across an excellent (and free!) piece of software here;
It is a real-time generative composer and mixer, designed for live performance of midi information, and after only a couple of days messing around I can promise you that it is quite superb.
It will work on a standard soundcard with the expected limitations that such a device will offer, but I have it powering a Roland XV-2020 and hey presto! sounds like 808 all over the place!
Don't worry if you can't play a note as the generative engine takes care of everything, from the break-beats to the synth-lines, and all you need to do is just tweak the filters or chop up the beats and melodies in real-time.
The best fun I have had in ages!