I have a standard 20" bushmaster A1 HBAR, and now that I can finally print 1.5 MOA groups at 100 yards I feel I finally "deserve" to buy a scope. (Wanted to learn to use the irons first.)

First, my requirements:
I want to buy one of those nice, short looking scopes to mount on top of the carrying handle. I need to retain use of the standard Iron Sites. I would like a scope that can do CQB work (geared somewhat towards target acquisition), but can also take shots out to 300 yards at a man sized target. I'd like to be able to shoot MOA (Even if the gun can't...) at 100 yards as well, but I am not too concerned about true long range accuracy. I won't be doing any benchrest competitions with this; mostly want some for shoter range (50-300 yards) paper killing, and SHTF CQB type scenarios. I also enjoy hunting cans and bottles, of course.
I know the 20" isn't good for CQB type work, but I will be building another 16" AR-15 at some point. While I will be using this scope with my current rifle for awhile, it's really that rifle I'm buying it for.

Now, that all being said, I have some questions that I couldn't seem to find answered.

First, mounts. What is all this ARMs stuff? Do they fit Weaver-style rings, or what? Is quality important? The $20 aluminum scope mount from bushmaster's site seems to be what I want, but what am I loosing at that price point?
I also might want to install a forward pistol grip at some point, so should I consider one of those railed hand guard type things? Wish someone had an FAQ on mounts and rings for the AR-15.

Second, what is this wiz-bang BAC things everyone keeps talking about with ACOGs?
I was looking at the EOTech Model 552 HDS Sight on the bushmaster website, and I could probably spring for that. But I don't understand what all the fancy HUD stuff does that a regular scope can't. Does it have a ballistics and trajectory computer in it, or what? I guess my real question is, what the hell do all these fancy ACOGs and EOtech scopes do, and why the heck do I want one? I thought a 'red-dot' scope was just a scope with a red-dot projected on the reticule...

As for the scope, I very much want to keep the price below $400, less if possible. I am seriously considering the Aimpoint Comp, and I have seem them with 2x instead of 1x magnification. Would this fit my distance requirements and still be acceptable for CQB? Does this scope work well in low-light situations?

Anyway, thanks for any help. Any other information you would like to add would be most appreciated, as would any links to primers, faqs, etc.