your looking into a bike that doesn't really work well.  you said you are going to race cross, xc or whatever, but really a mob would suck for that.  Aluminum hardtails work for MOUNTAIN BIKES, but are usually a little tall for a good street whip.  The mob is a great street bike but isn't that awesome for riding off road, you can do it, but the angles are off.  the steel head is designed around running a dual crown fork if needed, so the head angle will be pretty slack for a street bike too.  a real street bike is tiny, and should have a 12-14 inch seat tube max.  look for chainstays around15-16 inches at the most, and never run a fork bigger than 100.  that being said, if you want a freeride hardtail, which is less of a bmx bike, you should be able to run it geared or not, and have it a lil taller.  it won't be that dialed for street though, but you can run a longer fork.
i can hook you up with a basically new brodie holeshot frame, sized small 14'', painted custom copper, and a marzochi 4x fork for about 600 if you want, that's about as utility hardtail as you're going to find.  lemme know.