I have a new Bushmaster XM15 withe the A2 carry handle and a 20" barrel.  I like the red dot scopes, but I know nothing about buying one.  Can you recommend a scope that is not super expensive, but that will be good at 100 to 200 yards?

Most people here prefer the Aimpoint for red dot scopes.About $350 or so.I've got one and love mine.It's got a 4moa dot.At 100yds the dot will be 4" big against your target,at 200yds it will be 8".So 200 yds would be about the realistic usefull range for an Aimpoint.

I could spend all night telling you about different options.Just hang out in this forum and you'll pick up all you need to know real quick.

Aimpoint [:d]
Check out equipment exchange, good deals can be had there.
That's what I'd get, but if you want something inexpensive, I hear Tasco ProPoint is very good, much better than the BSA/Leapers POS.

I bought a ProPoint3 CMP...it's my stopgap until I can afford an Aimpoint.  It's got a very bright 10moa dot...not meant for target work, but it puts the round where you point it, and it's very easy to see, even on bright, sunny days.  At the highest setting, it's very hard to wash out...you sorta have to look straight up towards the sun...which you shouldn't ever do lol.  

I paid $90 for it at Midway...it's head and shoulders better than the $30 BSAs you see..but even they would work for a while if that's all you can afford.