[nst7]

The computer processing and ram will not make any difference in improving the sound quality. All they will do is allow you to perhaps run more instances of VI's and tracks, with less errors, etc.

Also, the type of interface will not make any difference in sound quality. There is no sound advantage just because something is a PCI interface vs. usb or firewire. There are crappy $100 soundcards that use PCI all the way up to the HD system.

Also keep in mind that jumping to MOTU will not improve your sound quality as their interfaces are no better then the Protools LE line.

The only way to improve your sound quality would be adding external A to D and D to A converters, and the good ones cost some money. In doing that you will also need to add external preamps for your mics, except in a few cases where the preamp and converter are combined in one unit.

For VI's, since they're already in the computer, your main thing will be the D to A conversion for listening back.

Also, you mention you're not happy with your VI's. Which ones are you using? The ones that come with protools are not bad, but not the greatest. There are plenty of 3rd party ones that are very good.

Maybe you could tell us more about what you're trying to do, what kind of music, how many inputs you need, etc. We could make some recommendations.