You need a low latency audio interface really to make it work in a live situation. This needn't cost a whole lot - in the band I used to be in, the keyboard player had a fairly old Toshiba laptop with an Edirol UA-25 audio interface. He loaded VSTi's into a rack system (not sure which he used, there are a couple out there, including free ones). The output from the audio interface goes directly into the house PA. He was also able to load other effects (he was a guitarist in the band too) that he put his guitar through. It worked (still does, I'm just not still in the band) really well and is super reliable.
Mac's have inherently lower latency than PC's in their built in soundcards, but Mac or PC, I'd look into having an external audio interface instead.
If you want a free VSTi rack host, look into Plogue Bidule. Lots of others listed here -
Asseca