l have a 2001 BMW 325i. l was wondering what parts add performance to it.

If you want to add performance you can do the following:
Buy a supercharger for more horsepower
Buy premium gas
Upgrade brake pads
Upgrade suspension for better handling
Get the ///M wheels

That is about all l can think of.

Contact Dinan. They make the best parts for your BMW and they will not void your warranty. They are very expensive though.

The two previous r correct, but l can tell u some better stuff.
Visit the website bavauto.com & u will find anything u need, & a little more.
If u r looking for a little extra umph, (without breaking the bank buying Dinan stuff, which is overrated in my opinion), a catback exhaust (remus, scorpion, agency power.), new air intake (conforti, dinan, AFE, K&N), underdrive pulleys (rogue engineering), ignition coils (bavauto.com), & headers will give ur car more power for longer in the RPM range, & give it a nice, aggressive sound. If u r looking for big power though, forced induction is the way to go, but maintainence will go way up. From all the things l mentioned previously, u could expect to reasonably see these gains at the wheels: Cat back- 5-10 HP, Intake- 3-6 HP, Pulleys- 8-10 HP, Ignition- 1-2 HP, Headers- 5-10 Hp. Those may not be overwhelming numbers, but when combined ur car will feel sharper, sound nice, & have a wider power band which is even better for everday use.

For all around performance, some drilled & slotted rotors not only look nice, but they also improve braking, as do stainless brake lines; which also make the pedal feel better & stronger. Performance brake pads (green stuff, hawk), & ifyou really want to dish out some money (if u plan on bringing it to a track), a Brembo kit. Although the effects r minimal at best, lowering springs & front diffusers will help. the lower ur car is, with more downforce, the better handling u will have. Another huge factor in performance is weight, which makes or brakes a lot of cars out there. Carbon fiber parts cost a lot, but they will cut down on weight. Another overlooked factor r ur tires. tires make a world of difference. You can really feel the difference in grip just by changing the tires to something more sticky, like Toyo Proxes, (which l put on my 98 328is), they really bring ur car to life on the twisty back roads!

***Another thing l forgot to add: If u r willing to cheat the law a little bit, magnaflow high flow catalytic converters gained my M5 an extra 8 Hp to the wheels, with the stock exhaust behind it. In order to keep ur check engine lights off though, u need to do a little custom work to cheat the O2 sensors.