Do you have any parts yet?
I can give you a good list to start with at least, just from memory:

Top Case inc. Trackpad and Speakers;
Bottom Case;
Front Screen Bezel;
Rear Screen Bezel;
Logic Board;
Battery Charge Board;
AC/Sound Board;
CPU Module & Heatsink;
PCMCIA Card Cage;
Airport Card Cable;
LCD Panel (There are two different ones, they might use different displaycables);
Display Cable;
Inverter Board;
Inverter Cable;
Modem Board & RJ11 Cable;
Airport Antenna Cable;
Microphone & Cable;
Optical Drive Module;
Hard Drive Cage & Cable;
Clutch Cover with Power Button;
PRAM Battery;
Port Cover (usually with the bottom case);
Heat Shield;
Rib Frame (often found with bottom case on eBay);

Obviously you will need a hard drive, RAM (PC100 or 133 SDRAM) and your choice of optical drive to fit in the module. And screws to hold it all together.
If I have missed out any parts, they will mostly be included with one of the other parts on my list.

Upgrade Options are as follows:
G4 CPU up to 550MHz or so;
G3 up to 1GHz or so;
RAM up to 1GB;
Hard Drive up to 128GB I suspect;
Will happily run a DVDRW;
PCMCIA Expansion;
Not sure if you can overclock the Rage128 GPU or not;
High-Capacity Battery;

Should get you started at any rate.

Probably would be too pricey at the moment, and the biggest one I found was only 1GB, but I have been wondering about using a PCMCIA flash card to boot OS X on an older PowerBook. I figure 8GB of flash would give you a nice big, fast scratch disk.