Ok guys, it's been so long since I raced road bikes I can't even remember my sizes.
I think I road a 52cm but could be wrong.
I'm 5'10" and 31" inseam. What size would you suggest on the mountain bike scene?
Personally, I ride a 17.5". It's a more compact frame and allows me to take tighter corners faster and ride tighter trails in general. For longer distance I would recommend a 19" frame, it's what my brother rides and from the rides I took on his bike greater distances are much more comfortable than on my bike.
He has a Trek 4900 and it's a beauty for sure. I like my Giant. Stay away from Specialized or Trek though, and that's not to say that they aren't excellent bikes, but they are overpriced for the technology you get. You can spend much less on a Giant, Gary Fisher or Kona than a Trek or Specialized with equal technology. Read reviews, check out MTB Magazine... There's a lot of information out there for any bike you consider (just like with motorbikes). You don't have to listen to any advice on brand though because that's a personal choice thing.
As far as frame geometry goes, Giant have excellent climbing capabilities as do Gary Fisher and Cannondale. Giant's only downfall is their weight, my bike weighs about 30 pounds and it can be a bit too much occasionally. Most mountain bike companies have a lifetime warranty on the frame so if you come down off a drop and the frame cracks, you're covered. Since it's not a moving part they'll replace it with the same or equivalent frame.
Braking is a pretty important factor to consider too. Disc brakes are very nice and have great stopping power, but the traditional rim brakes will suffice perfectly. Keep in mind they will weaken the rim over time, where disc brakes you will replace pads and discs and your rim will last longer. Discs can make some pretty bad squeal if they get dirt or water though.
I guess I should be asking what kind of riding you wanted to do though? Because if you're planning on doing down hill you're looking at a higher priced full suspension piece of equipment than a more efficient hard-tailed cross country bike. Also, unless you spend upwards of $2000 a full suspension bike will likely disappoint. A good hard-tail can be had for $500
Unfortunately today was pretty crappy out and I probably won't get another riding day in this year to take pictures. The rest of this week is supposed to be nice though and if I can get out on Friday I'll get some terrain pictures for everybody. There are some pretty cool technical areas to cross on the trails here.
Edit:
http://www.mtbr.comhttp://www.mountainbike.com