Any advice on buying an interface/thru box for hooking up a cardiod microphone to my mac to add vocals to a soft synth? A USB or firewire gadget or whatever.
There are plenty of products that would work for you. I grabbed the links for a couple, but depending on the features you want and how much you want to spend, these may not be the best choices.
1 and 2 are probably your best bets. The Emagic hardware doesn't include preamps, which are a necessity when using a mic. Let me know if this helps you and if you need any more information.