your owner's manual has all of the recommended maintenance intervals listed. oil is the lifeblood of your motor, ALWAYS change it, charge the stuff on your credit card if you have to.
here's some stuff i'd pay attention to in the immediate future:
new plugs and wires, and check timing
fuel filter
air filter
distributor cap and rotor
valve adjustment (every 15k miles)
tranny fluid
coolant
new belts aren't needed til around 90k miles.
best way to get rid of the a-pipe rattle is to get an aftermarket header. you'll save money over a stock one and you'll gain a few HP.
other problems many GS-R's have are springs popping out of the clutch disk, leaking clutch cylinders, leaking radiators (where the aluminum meets the plastic endtanks), and leaky cam seals near the distributor side. my 95 also eats through brake rotors pretty fast, and i needed to replace the brake master cylinder at 80k. some little stuff that bugs people are broken antenna masts, rattling hatches, and clunking noises from the rear suspension.
i'm not a big fan of engine modifications on GS-R's unless you open up the motor or go with forced induction. from experience, the typical I/H/E is not cost effective and will make more noise than power. on the other hand, suspension modifications and a good set of tires will make a world of difference.
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