Hey mate, i had a 97 NA a while ago, tried to do some tuning and found the best mods are:
stock airbox with panel filter, high flow cat with an exhaust if you like the sound and keep your maintenance up (oils, plugs etc)
jst as a downer, i pop'd in a unichip and dyno tune to push out 78awkw which was quite dissapointing but tuning does pick up about 10kw in the low to midrange area if you have the cash to spend.