As always, Gary is pretty much on track here. Swapping G4 mother boards from different models (like an MDD into a QS) is very challenging; I am not saying that you are not up to the task, but it is advised to leave QS and MDD boards in their respective units. Most of our core members have done some of this crazy-ness in our youth, myself included, if you have endless time and an enhanced workshop than go for it ! But in the end, you might see the master design of each G4 model is sacred...
That being said, since you have a 733, there are a few things to keep in mind
1) If you plan to use large IDE hard drives,
make sure your QS MB is part number 820-1342-B From our site:
Power Mac G4 QuickSilver 733, 800, Dual 800, 867, 933, and Dual 1 GHz with logic board 820-1342-B (QS logic board 820-1276-A will NOT work and peak out at 128 GB)
Swapping out a QS MB that doesn't support large drives with one that does is very straight-forward
2) The 733 QS CPU is a total dog with only 256K cache and will under perform a 500 Mhz sawtooth when it come to audio
From our site:
Again, be aware that you must check out the machine’s technical specs very carefully to insure you get at least 1 MB cache and that the G4 can boot directly to OS 9 (all Intel CPUs are automatically out of the question). Also, be aware that more expensive dual processor G4s may not be any faster since most OS 9 applications will ignore the second CPU all together. If you follow my guidelines you won’t need a dual processor. I have personally been involved with audio projects that have over 60 tracks of 32 bit audio with effects on a single core G4. By the way, some of the fastest OS 9 Power Mac Towers (Quicksilver G4 933 MHz and MDD 1.25 GHz 2003)
So swap out the CPU if needed for Audio recording or graphics design.
Use a 933 single if you are staying the world of OS 9
or a Dual 1 Ghz. if you are going to dual boot to OS 9 and OS X.