I'll list what i can remember right now (i have the 325is)
Exterior:
325is has a rear lip spoiler and BBS alloy spoke wheels
325is has a lower Front spoiler and foglights
Engine and Drivetrain:
-325i and the 325is both have a 168 hp and 164 lb-ft torque engine using the motronic 1.1 fuel injection system.
-325is has a limited slip differential (which i have to say is very good to have)
-325is has a Getrag 260 transmission, which is much better than the ZF tranny used in the 325i (i think...)
-325is has a stiffer, sportier suspension which makes a huge improvement in handling
Interior:
325is has an M steering wheel with no air bag, and leather interior with the M3's sport seats which is also another reason to get the 325is over the 325i. 325is also has a 6 speaker sound system, versus a 4 speaker in the 325i.
I cant think of anything else.