hey man, here's some solid advice from a non-gear snober.
some of those bikes are worth upgrading, some are not. being that you only spent 20 bucks and you feel like making some upgrades to the bike, I'll do my best to give you the most solid advice i can. before i go into this, you need to understand that to upgrade this bike to any kind of halfway decent, offroad worthy level, you're going to have to spend the same amount of money that could buy you a better, decently equiped mountain bike used from craigslist, ebay, or any of those sorts of methods. however, I'll still go into this...
the frame on this bike looks like it weighs quite a bit, and the rear swingarm looks to be made of steel while the rest of the frame is aluminum... the stroke of the rear shock looks VERY short meaning rear travel is very short, this may make finding an upgrade pretty tough.
step 1. upgrade that fork! i reccomend the rockshox dart series of forks, pretty cheap and they give a lot of bang for the buck. I'd say go with a Dart1.
step 2. is your seat comfortable, is the seat high enough? are you comfortable on the bike and does it fit you properly? do a search on google to find out how to make sure the bike fits you. if your seat is uncomfortable, or the seat post is too short to get the seat to the correct height, buy a new seat and seatpost. go to your LBS, not target or walmart, the seats they sell there will be no better than what is on the bike.
step 3. upgrade the rear shock. check with X-fusion for new a rear shock, or look online on ebay for something a bit more "name brand" as was mentioned earlier, you need to measure the "eye to eye" length of the rear shock, that means from center to center of the mounting holes on each end of the rear shock (if you dont have a mm rule, measure in inches, convert the fractional measurement ie: 4 1/4 = 4.25 and then type this in google "x.xin to mm" once you have this measurement, you'll also need bushing size measurements to make the shock fit the frame, a lot of times you can just re-use the ones that came on the old shock, unless the inner diameter of the eye is larger than the outside of the bushing to press through it, in which case you need new bushings.
step 4. these upgrades will probably have cost you over 300 dollars with ease by now, but if you want to keep going... we can. the crankset and bottom bracket are probably another weak link on that frame, if light duty offroad riding is all you plan to do, just put decent pedals on the bike, if you're happy with the crankset, pedals, etc... leave them alone.
step 5. by now your wheels are probably out of true, rubbing against your brake pads and the like... again, it was mentioned earlier, go with mavic, or pretty much any other name brand wheelset. there are normally some pretty decent sales on wheelsets. at this point your closing in on 400 or more dollars....
however, your bike is almost halfway decent now, but if you find yourself REALLY enjoying mountain biking, do yourself a HUGE favor and get a nicer bike. you might be able to re-coup some of the money spent by selling the upgraded parts on ebay, but you'll never get back what you spent.
I made the mistake of upgrading an unusual target frame... i spent a lot more than i anticipated and in hindsight, i should have looked for a used full suspension frame and built that up instead. i dont nessicarily regret building up this frame, but if i had to do it again I'd probably not do the same thing. my advice to you sir, is dont spend too much money on this bike, go in the order i went in, and upgrade till your happy, or like a few other folks said, ride it till it breaks and then buy a good used bike...
bottom line: just enjoy riding your bike, being outside and getting into the sport of mountain biking, its a hell of a lot of fun, dont let the gear snobs get you down...
I shoulda bought the wife a new fridge instead PWAH! And besides if anyone of you that bad mouth economy riding fellow bikers with a three thousand dollar bike were to be riding in my neck, I would dust you on your shiney refrig expensive bike anyway with my toy bike 
