Hey back to the OT. I think OP should be more worried about his monitors, as, and not trying to make assumptions but 99% of the people I know who use VSTi's as their main sound sources tend to also monitor on headphones and computer 2.1 systems and in untreated bedrooms through their sound cards, and as an interface could help just as much as a good sound card the money may be just as well or better suited to a good set of monitors or some acoustic treatment,
The other thing that struck me was the word Mac, now, I use a mac at home and at work and unless he has some embodiment of mac desktop soundcard installation could prove painful if not downright impossible. Apple really aren't that fond on you upgrading their products, (except the mac pro's which are handy enough).
So the question is OP Do you have a Mac Pro, Decent monitors, a well taught out control room or are you taking the headphone outs and mixing on headphones? or thru speakers in a not so well treated room?
In the first case I'd suggest a new soundcard definately, in the other instances I'd be more worried about monitors and acoustic treatment.