I would recommend one which can be separated for cleaning and is made of SS where it contacts the food. The plastic ones pick up flavors especially if you use it in both hot and cold.
I have the Braun MR 5550 but I've had it a few years so it's probably been replaced by a newer model. I got it at BB&B for about $100. 400w - very powerful. Mine came with a bunch of accessories including a nice polycarbonate pitcher with silicone seal cover. Great for OJ, etc.
Update: Just saw it on kitchen_dot_com for $50 (Thanksgiving Day special)
I have the Cuisinart; I think it's what Cook's Illustrated recommended. I love it. It comes with a variety of attachments, one of which is perfect for smoothies. I don't make them often myself, but in general I've been incredibly happy with its performance as a mini food processor, soup blender, high-powered whisk, etc. It's not cheap, about $90 in Bed Bath and Beyond, but you can usually get a 20% off coupon there, which helps. Maybe you can find it cheaper elsewhere. I'd recommend it.
I am on the same boat as you and been doing some research on this. The kitchenaid khb300 is on the top of my list and been waiting for a price drop. Its about $80 at the moment.
If anyone would kindly share any expereinced in this model or any others, we would appreciate it!