Assuming the car is in good running condition without the need to spend money on tune up or mechanical parts (i.e. brakes, struts, A/C system, other electrical devices), I really think that the Progress rear sway bar is very well worth the money as a first mod for handling purposes. Since you mentioned having an intake filter as a performance mod, I'm also assuming you're just planning to go N/A so the next mod "worth the money" would be a 4-2-1 header exhaust manifold mated to a custom 2.25" full mandrel bend exhaust system welded to a straight through muffler (I recommend magnaflow - it's very quiet compared to others I had). That should satisfy your best bang for the buck needs for now. Hope this helps and here is also a mod list that can be found here if you want to spend more money.
http://www.sr20-forum.com/all-motor/40-newbs-basic-na-tuning-guide.html
You can always slap a turbo on there if you really want to go fast for real cheap