Go with Amplitube. Awesome simulations of AC-30's, Marshall 800's and other well known makes. There's also a ton of factory settings that imitate specific artists. The only problem with computer based simulators though is you need a fast computer or you get a delay. I'm using a Mac G3 with Cubase and have to record leads straight line in first, then put the plug-in on the sound later. Kinda inconvenient cause you have to imagine what the finished sound is gonna be like while you're playing this dinky line-in sound, but you can spend as much time as you want later trying out different amps until you find one that fits the song.
Another unit I've mentioned here before is a RedBox Cabinetulator...a small box that plugs in the line through of your effects pedal. The advantage there is you can play live while getting the sound you want, and with some careful positioning of your computer speakers, get a great feedback sustain.
Basically every tune I posted up until last December is straight line in through a Boss ME-5/Cabinetulator. More recent stuff is likely to be amplitube or a combination of the two.