Give us some more info. I assume you have amps you're happy with for both guitar and bass? Do you have a mic for them? The Shure SM57 is a cheap, and probably has a better consensus than any other mic at being decent on guitar amps.
I assume you have a computer - does it meet Sonar's requirements - do you have a soundcard you can record with? Is sound quality enough of an issue that you need to put down $100+ on a better one? (ie, a Sound Blaster will work, but it won't be beautiful).
You'll probably need a seperate pre-amp, like
MAudio Audio Buddy (another well-though-of cheapy - I use one). If you get a decent audio interface to replace your soundcard, you may be happy with its internal pre-amps, and not need to worry about this.
One way to get going is to get the SM57 and Audio Buddy, attach the mic to the pre-amp with an XLR cable, and attach the output from the pre-amp to your computer with a instrument (aka "guitar") cable to your soundcard in port (you'll proabably have to get a 1/4" to 1/8" adapter from radioshack to connect the cable to the soundcard).
But, I'm still just guessing at what you need until I know exactly what you have, and level of quality (and, of lesser importance, how expandable you want your rig to be).
