HashiriyaGDB, seriously ditch that attitude. Just switching to a MT doesn't make your car super fast. It's not like getting a MT and all the other necessary parts are all that cheap either. As it can run from 0 to 800 bucks. Additionally it's obviously not helping you get anywhere regardless of how fast you think you are with your 5 speed.
As for my thoughts on cheapo mods that'll do some work would be very logical. The simple bolt on stuff. Usually the cheapest, and usually also the easiest to do. Advancing the timing can also have adverse effects in the long term with aggressive advance as you'll need to use more expensive gas and that'll add up considering you're a student.
My take on the situation would be air filter/intake. Replace the stock airbox with a conical filter like the apexi cone filter (60+ I've seen for apexi specifically) if that's within budget. If not, the ebay brand stuff is decent too (usually 10-20 with the cone adapter, which will require one hole to be redrilled). They may not have the name brand associated with it, but they work. Replacing the piping with something not plastic would help as well for the intake. If the injen systems are within budget, go with that. The ebay brand stuff for the piping is advisable to avoid as they're usually missing one or so nipples for your vacuum lines.
The next step on my list would be expensive, it's the cat-back exhaust. If you're looking for something cheap, you can get a cost effective but decent cat back for 300 bucks+ s&h like the Uras RS from raretrick. If you've got more money to kick around for a more worth while investment, try to shoot for Apexi, Blitz, Greddy, HKS or my personal favorite GP Sports. They'll cost you a good bit, probably around 500 or more with s&h.
Suspension wise, try bushings. Small but effective. Chances are with the age of the car, the bushings for the suspension are more likely than not old and tired. Most common of them are your tension rod bushings. It'll make for a more fun ride. If you can get them, get sway bars as well, they're a more worthy investment than strut tower bars, but also cost a bit more. Check out
http://www.pdm-racing.com for whiteline bushings (they may still be having the special for the complete whiteline bushing kit for 200-300 bucks) and you can ask around for sway bars. My personal recommendation would be either JIC or Cusco (400-500 for both f/r). Many people also use whiteline. Look into those.
Safety wise, check out brakes. Rotors that are worn and warped are common problems I see on many s13's and the brembo discs are a good investment. I got mine for 180 for all four with slots and cross drills. Don't get the ones with the slots though, as they'll chew out your brake pads. Trust me, I've got them. Stainless steel brake lines are a bit pricey and may not be as good in terms of bang for the buck. I've got them too. Perhaps the most effective brake upgrade would be to do a brake seal job for both front and rear calipers. Might as well get them working to par before upgrading some more. Cheap but effective.
Perhaps the most overlooked for getting lost power is a simple tune up. Most likely than not, unless your car was recently maintainenced, will need some form of TLC to get it back to making the stock power. Easiest way to do that and get better mileage while you're at it is a tune up.
I know that's a long list of stuff, but I too am a college student with a budget in mind. There's much to consider and it usually helps to have someone who's gone through those same basic mods first and have their input on it than to just dive right in and waste money in the long term with regrets in retrospect.
Good luck with your mods and hope all goes well for you.