1. You going to get more power from the 3000+ chip in #1 if your going to be gameing. AMD chips have always been the better choice when it comes to that.
2. Same as above.
3. None of the Ad's you posted mention what motherboard's are in the system, but I would assume most of them are generic brands. I don't think you can expect to find a top of the line board in any of the systems.
4. #1 does mention that it has on-board 7.1 sound. Which is fine, no need to make any changes there. It doesn't mention the sound on the other 2 systems, but I would assume their on-board as well.
5. As far as them being compatable for upgrading, I don't know. Some manufacturer's create propriatary parts specific to their systems. Which means the only way you can upgrade the system, is by going directly through them, which sucks. I think #1 is your best bet in that area. I don't think I would trust HP or Gateway in that area, but then again I have never owned an HP or a Gateway, and I don't know how they build their systems, and I only make this assumption based on what I have heard from other people. Stick with #1.
6. Cooling is not mentioned in any of the systems. They should be fine. You can always easily add case fans.
7. The RAM manufacters are not mentioned in any system, though you could expect it to be generic as well.
8. I don't know what you mean by "Data Bus Speed" , sorry can't help you there.
9. #1 is your best bet for a decent gaming system, and will probley be the most trouble free when it comes to upgrades, and gaming.
10. Depends, if your really want your gaming experience to be good, and you have the money to spend, get a good LCD monitor. Don't settle for a cheap, on sale Wal-Mart LCD, unless it doesn't matter to you much. (I say that cause I bought my "Cheap", "On-Sale" monitor at Staples
. A nice 19" Flat Screen CRT would be a good choice as well, you get great quality, without a huge blow to the wallet.
11. If your thinking LCD, your going to want to look at the response time, the lower it is the better. I would say anything 20ms or lower, and you should be in good shape, but people can correct me if im wrong.
12. I can't think of any off hand, but I am sure people know a few you should be asking. Good Luck
*EDIT* I didn't see the the additional specs you posted, so some of my information may be a little off.