one with 2 wheels, and the words dirt, jump or park in the title. Realisticaly the only difference will be the tyres, if you want the right tool for the job get 2 sets of rubber, if not just get slicks as you wont be on the dirt unless its bone dry anyway.
~£250 and upwards gets a good enough new bike, the same ammount 2nd hand gets soemthing a bit more special.
Go for cartrige bearing hubs with cassetts rather than screw on free wheels, 3 piece cranks and a full cro-mo frame and fork (no hi-ten rubbish). Deacent rims should be up there on the list of things to have too.