The 630 has one 72-pin SIMM slot that can take anything up to a (single banked) 64MB SIMM and reputedly can even work with some 128MB SIMMs (very rare). Your best bet is to find a 72 pin 32MB SIMM as most of those seem to work fine.
The hard drive is of the IDE variety meaning that it is very easy to find cheap replacements. There may well be an upper limit on capacity in the 630 but to be honest a 1 or 2GB hard drive will be ample for most System 7 setups. As for the OS, I would strongly recommend System 7.5.3 / 7.5.5 - both free from Apple. The Quadra 630 will fly with that and 36MB total RAM and there are loads of applications out there that require 7.5 or up.
