check out the Poulan Pro PP5020AV, it is a beast of a saw, can be found for $200 or maybe a little less, and if you read on Arboristsite.com
you will find that it is reasonable well regarded. 50cc, over 13000 rpm, and a 20" bar.
You may also find that there are some other inexpensive saws that don't totally suck, too. The chain that comes with it is a safety style chain, but it is a Vanguard, so it cuts pretty good. I have torn up more than a few trees with mine, and a BUNCH of bamboo. Just don't hit a brick while trying to cut out some crappy shrubs. a few tries still at sharpening the chain, and let's say that it is still a long shot from perfect. Think I may finally give up and order a new chain, also a 16" bar and chain - more in line with what a lot of pros expect a 50cc saw to run anyways.
My next saw, budget permitting, will be a Husqvarna 562XP, or the Jonsered equivalent.