lots of sifu's here who can give very good advice. but the rule of thumb is that with great power comes great loss of $$$
like what unker 535i said, change to 17' rims and you will feel the difference. 18'" is only when you're running like unker 535i power level.... :O
antiquer.....M50B20 got a lot of potential. plus u're a manual box. but to just chip it, wont be much of an increase. a good bang for buck with little/no maintenance, is to stroke to 525/528/530/532. an increase in CC, with a chip, freer flowing exhaust, etc etc would be a nice gain, plus no worry on maintenance. cost wise, wont be between 2-3k, will be more. you can do it in stages. thats the fun part.
otherwise just turbo it like fandango said. if you're happy with 250rwhp, you can just slap on a turbo, get it tuned and you're good to go