If you'd like a compact machine, the Quadra 605 variants (Q605, LC475 or 476, Performa 475, 476) are great.   While the stock speed is 25Mhz (as fast as the Q700) they can easily be upgraded to 33 MHz and taken to 40 MHz with somewhat more trouble.  Also, with a 128MB SIMM, their memory can go to 132MB or almost twice what the Q700 will do.    The Q700 has more Video RAM (larger monitors and/or more color depth) and two NuBus slots and ethernet built in.  The Q605 has an LC-style PDS slot.  Neither has room for a CD-ROM drive.

A Q700 logic board will fit into a Centris or Quadra 650 case.   Then you could have an internal CDROM drive.   I think a 7100 case will work as well, but I'm not sure if the rear slots line up properly.    The rear ports on the Q700 and Q650 match up properly, if I remember correctly.  Of course, that wouldn't give you a tower.  

I bet that the Q700 logic board will fit in the Q800 case as well, because the Q800 logic board is identical to the Q650 logic board.  So if the Q700 replaces the Q650 and the Q650 and Q800 are identical, the Q700 should fit into the space where the Q800 was...

So, if you want a tower, and don't want something as big (and rare) as a Q900/950, perhaps the thing to get is a Q800 and install a Q700 logic board (assuming the Q800/650 won't do 24 bit addressing).

ArmorAlley:  Seriously, you got your Q605 to 50 MHz???    Did you have to replace the MC88916 or MC88920?  Did you use the DW70 or DW80 variant if you did.  What speed VRAM did you go with?