First off, make 3 hours of time, and take the free practice GRE test on the ets website. That will give you a good sense of how well you're going to do.

You say two very different things in your post - that you've only taken two "real" CS classes, and that you know basic algorithms / data structures. A lot of what comes out of an algorithms or data structures class is the vocabulary surrounding those topics, and my experience was that the vocabulary is a significant part of the GRE.

The worst thing that happens is that you're out a hundred dollars for the registration fee, so you might as well find a CS GRE prep book and start studying. It is definitely more doable to cram for the CS GRE than for the Math or Physics tests.