I'm going to get a mountain bike soon. I used to race dirt bikes, and I ride/rode freestyle bikes until my shoulders got too bad. But I just moved to a town that has tons of MTB trails. I was hoping to spend under $1000.
Thats reasonable. There are alot of good bikes, both fs and ht, in that price range.
Basically I don't know what I want.
Disc brakes? Mechanical or Hydraulic?
This depends on the trails. Lots of muddy or wet conditions, or very long downhills, then discs are king. I prefer mechanical, because if you pop a line on a hydraulic brake you're hosed.
Hardtail? Full suspension? Can I even get full suspension on my budget? Do I even want full suspension if I will use the bike around town a lot?
This depends on the trails in your area. Lots of rocks and technical riding might dictate an FS bike. Lots of climbing might dictate a HT.
You can get either in your budget range, but you can get one freaking seriously killer hardtail in that range. You're limited on hardtail choices, and probably won't find air-shocks in that range.
All manufacturers pretty much have their own proprietary suspensions, right, (aside from forks), or is there some degree of standardization?
Sort of. The Specalized design is one of the best out there, and is widely copied (through royalty payments, of course). This of course only goes to fs bikes. Hardtails are pretty much universal, just depending on frame design and how it fits you.
I especially want something that can be upgraded. I have probably $1500 in my little kids' bike, and I'm familiar with the phenomenon of buying a complete bike, upgrading everything but the seatpost clamp, and ending up more expensive than you could have just bought a nice bike. So something with a frame and suspension that I will be happy with, even if it has crappy parts is better.
Any decent mountain bike can be upgraded, or parts are easily replaced when damaged or worn.
I ride an inexpensive FS bike, but I also got a deal on it. I'd suggest a HT in your price range, you'll find the best components and front shock for the buck.