You can run Cubase+GigaS(Studio) on one computer w/one soundcard. But nemesys recommends that you use 2 computers: one is the trigger(sequencer) and one is dedicated to just GS(t). This should be the best.
You can goto the nemesys-site and check out the supported soundcards. I myslef have a Delta1010, and it works perfectly. 10in/out.
Midiman soon supports a special feature: You can have Cubase+GS running and you can use have low latency on both: CB+GS, this should kick some asses. But there are a lot of other soundcards which you can use for these proggies. You need a mixer w/several ins/outs, as every proggie need it\'s own outputs.
Go and get an Duron(maybe <750) they are cheap, and get the Asus A7-v, you can overclock your duron up to 1050 mhz!! and it works. Get min256 Megs of ram and a fast IDE drive. scsi offers better performance, but these drives are expensive. (got a Atlas 10kII, 28 megs/s read/write). GS loves it.
If you want to get cards from echoaudio (gina/mona/darla...), know that they usually don\'t support athlon systems. Only if you get a mainboard w/KX-133 chipset.
If you want to know more...
Mixtreme cards have provided this feature for well over a year. You can run GSIF, ASIO2 and MME drivers into the Mixtreme DSP mixer. This allows you to record GS into Cubase VST with no additional latency, even using the real-time effects on GS.