I agree with Brakkum here -- if you've got the cash, digital amps (and also solid-state amps, although this is more of a debate) are inferior tone-wise and responsiveness-wise to tube (valve) amps.
Digital amps/pre-amps tend to be "cold" sounding, can be somewhat sterile. Most guitarists prefer tube amps for a warmer, more organic sound. Not that digital amps sound bad, per se, but that they're not usually associated with "good tone." The tube amps overdrive is gorgeous and natural sounding compared to a simulation of that sound (which is what you're going to get with a modeler). Petrucci's Mesa Boogies, for instance, are tube amps. Marshalls also make very nice tube amps. Other famous names are Vox (think Beatles tone) and Fender (Stevie Ray Vaughan and nice clean tones).
You're not going to find anything like this for under $200, and if you do, it won't offer the variety of sounds/tone palette that you'll want if you're making Dream Theater-esque music.
Check out Harmony Central's user reviews to get more of an idea of the strengths and limitations of what you're looking at. That place is invaluable for researching musical equipment.