Starting Out

I don't know that there are specific "dirt jump" bmx bikes, I would look for something with a little longer top tube (20.5, 20.6, 20.75, or 21"), and a head tube angle that's a little more slack 74.5, or even 74.0. Personally I'd stay away from a frame with a 75 degree head angle for jumping.

a bmx works great for jumps, they're reasonable priced, they're durable, easy to pump though jumps because of they are so rigid, they are really flickable, and they are easy to maintian.

If you want a 24" bike, there are a lot of options out there that you can look at (check the treads in this forum)

24" bikes are nice because you can put a suspension fork on them, which would diffinately be a little more forgiving to learn on. They are more expensive to build up then a BMX (although there are some decently priced options) 24" wheels, and suspension fork really shine on dirt jumps because they smooth everything out compared to rigid bmx bikes.

26" bikes are nice if you want to build something that can "do it all" bike. You can normally take the same bike you dirt jump on, and ride mtb trails.

I'd go with what scurban suggested. I believe the Felt base 20.5 is alum with a chromo downtube. Alum scares me.

the spec. fuse bikes have pretty good geo, good components spec and is pretty cheep. So bmx route, i'd go with that. As far as a mtb. the specialized p1 is cheep, single speed, and cromo. Pretty much what you want for DJ, and park riding.