I have the 30D, and for about a year it was my only body. Perfectly happy with it. The main reason I bought my 5D was so I could have 2 bodies in order to shoot weddings without fear of one failing and I'd be up a creek. I chose the 5D because of the full frame. Your 30D is a very capable camera. What are you looking to shoot that you feel the 30D is inadequate?
All EF lenses are compatible. EF-S lenses are not.
One thing to note is it is rumored that Canon is going to be coming out with an upgrade to the 5D early this year. If you aren't in great need of one, why not wait until either a) the new one comes out and b) the 5D price drops?