OK, keeping the stock BMW audio system with the original radio from 2000 there are the options I can think of:
First the very easy options :
1) FM transmitter - cheap, poor sound quality, no head unit control or display.
2) Tape modulator - cheap, poor sound quality, no head unit control or display.
3) Hack the tape input - cheap, work required to get decent sound quality, no head unit control or display.
After those things can get a little complex and depend on what system you have, I'll assume you have all the original components (C43 radio) and DSP.
Connecting anything other than an OEM CD Changer made before September 2005 will probably require an A/D converter to change the analogue output into the digital input required by the DSP amp. Even the OEM CD Changer made after September 2005 does NOT have the digital output. So before you go any further, check the existing CDC connectors, do you have a 3 pin and a small coax or a 3 pin and 6 pin? The small coax is the digital input, if you have this you will need an A/D converter.
4) Replace the existing CD Changer with another OEM Changer with digital output.
5) Replace the existing CD Changer with a new OEM Changer (plus A/D converter). The new CDC has MP3 playback capability.
6) Replace the existing CD Changer with an Aux input (plus A/D converter). eg Connects2 CTVBMX001. Quite cheap, good sounds quality, no head unit control or display.
7) Replace the existing CD Changer with a DICE Silverline (plus A/D converter). Quite cheap, iPod and Aux, good sound quality, head unit control of iPod, lots of reported problems, single line text display may not work with the C43 radio.
8) Replace the existing CD Changer with an Intravee II (plus A/D converter). More expensive ($350, including A/D converter, plus AI-Net device), good sound quality, head unit control, full text display on C43 radio, other 'interesting' features (vehicle info display, folding mirrors etc.).
Possible AI-Net options:
a) iPod interface with KCA-420i.
b) CHA-S634 MP3 capable CD Changer.
c) DHA-S690 MP3/DIVX/WMA capable DVD Changer. *
d) SIRALP-1 Sirius tuner. *
e) TUA-T550HD HD radio tuner. *
All the above solutions are plug and play except for powering the A/D converter with requires a switched power source and some of the AI-Net devices (marked with *) that require unswitched power. There are several other iPod type interfaces, Grom, Neo, Dension Gateway etc. Most have similar or slightly better functionality to the DICE, most have better reliability reputation but the DICE is the most common and thus may simply appear to be less reliable due to the number of units out there.
Hope that helps.