What kind of mountain biking do you intend on doing? XC, aggressive XC, downhill, freeride, aggressive freeride, etc?
I'm personally getting into freeride biking and slight bits of downhill. Currently, my ride is a crappy Mongoose D40r which will be cannibalized soon and upgraded with quality parts. I plan on a Marzocchi Z1 Bomber Drop Off front fork, an El Jefe rear shock, forget which wheelset but one that's incredibly beefy, rapid fire shifters, hayes disc brakes with manual braking for cheapness, probably clipless pedals ironically, and I might see if I can fit an 8-gear sprocket on the rear. The yellow is cool on the frame and the frame is unbreakable, so it's nice for hammering. Hopefully my bike will be completed in a year or so.
Also, depending on what kind of MTBing you intend on doing, you'll have to consider whether you want front suspension or full suspension. Generally, XCers go with front suspension to save on weight while freeriders and downhillers go with full suspension for cushion (also, if you're doing dowhill, the porkier the bike the better). Also, don't spend less than $500 on a new bike if you're serious about biking. $1,000 is even more preferable if you have the cash. If you're competitive, your bike is probably going to cost more than a.net's most beloved Ford Excursion

. The other route is picking up a reliable used frame for cheap and using that as a platform for upgrades. Mongoose full suspension bikes are great for this purpose, but don't expect to get a halfway decent bike if you get it stock.
I don't know, so this is my signature.