My suggestion before you do performance upgrades.... go to Napa auto and get a new water pump to replace yours when it breaks (it might break very soon, many 2.5L have a broken waterpump at 50-60k miles)... get a water pump with a metal impeller.
That should be your first mod... After that check the modifications section of this page for links to what you might want.... I'd do exhaust and then intake... one step at a time as you can afford it. After intake and exhaust, you might want to get a performance chip... those 3 things will transform your Mystique into a new beast.
Check the links section for pages for aftermarket parts... for example,
www.ctamotorsports.com and
www.spmotorsports.com might just have everything you could want.
Here's a cheap mod you can do first though... go to the FAQ section and read about the Throttle Hang fix (TH).... it is very easy to do and you'll like the throttle responsiveness when you do it... I live in a mild climate (no snow in winter, about 75-80F in summer) and used the TH fix as described with a 1/4" hole and my car loves it.
Welcome to CEG and have fun.
