Hey Chris
Dave gas made some great points but I've never had a problem playing PC tracks, and recording a live instrument to the PC.
In saying that, since getting my creamware cards I've had no latency issues.
Dave also mentions that they can't run VST instruments, there are however lots of synths, samplers, mixers and effects designed for creamware (a fair amount of which you get with the card), and this frees up your CPU for other tasks, including VST instruments. All in all this gives you far more scope.
The only reason I mentioned the 3 DSP card is that it would give you an idea of what they can do, without the massive outlay for the larger dsp farm cards. That and if you get one of these cards, you will still be able to link it directly to your 3 dsp card. With Creamware, you can use XLink to hook 3 cards up in the same PC. I believe that also covers 3 of the 15 dsp cards! Although be aware, the more dsps, the more ventilation will be needed, which means more fan noise, so you'll need to invest in an ultra quiet cpu fan and possibly a 2 fan PC case using ultra quiet fans.
Cheers
John