Max-out the RAM for development purposes, as a rule! For OpenGL, it's nice to have something that can easily swap AGP video cards for testing various configs; my Dual 1.42 G4 won't recognize a 16MB ATI Rage AGP card though, but anything greater seems to work.
I just acquired an old 233 MHz iMac Rev B which has an ATI Rage Pro Turbo and 6 MB VRAM. Hardware accelerated graphics is NOT supported on it though, so MacFOH refused to open its analyzer plug-ins (well, unless I enable it to use the software renderer).
I swapped the stock 4 GB IDE HD for a 40 GB 7200 RPM ATA drive with a 7.5 GB boot partition. The iMac had 32+32 MB RAM, thus I couldn't install OS X, but I pulled a pair of 256 MB SODIMM from a PB G4 800 and the iMac recognized them as 128 MB each (strange?) but I was then able to successfully install 10.3.8. I think the faster HD helps considerably, because the souped-up iMac seems quite usable now. :)