No one has mentioned the DR-880? This has got to be the best drum machine still out there, and it is like 5 years old now or something like that.
Anyway, this thing costs a few hundred dollars or so, but it flat out kicks arse! ... And you can even plug in your bass/guitar, and play with the bass and drums already in the machine.
Another option, that only costs like 100 bucks, is a program called BIAB (Band in a Box). All you have to do with this is type in some chords and BIAB does all the rest for you. The insturment sounds are very MIDI sounding, but the drums sound very good. This is probably the best virtual band there is on the market to practice with for the money.
Or you could go the "Reason" route too. This is like 300 bucks, but the learning curve to get it to do what you want is the equivelent of taking college courses for a year or two... not to mention that you will need some decent speakers and a good sound card for your PC as well.
Good luck!