Dual channel doesn't really increase performance that much.  Performance is still primarily based on latency, and dual channel doesn't really help in that regard.  Still, you'll get a speed boost, but probably not that much.
Do you need 3GB?  If not, I'd go 2GB.  But if you find that you're constantly running into swap space with 2GB, 3GB will give more performance than 2GB in dual channel.