Yes, basically. The only noticeable, and non-user-upgradable, difference is that a Q800 will always have 8MB of soldered mobo RAM, while a Q650 mobo may have either 4MB or 8MB soldered on. The only other thing is that if you're buying a board that hasn't been upgraded (or stripped), the Q800 board is more likely to have 1MB VRAM installed, while the 650 will have 512k.
I would say, absolutely yes. I've never had a need for more than 3 NuBus slots, and I've never found any need for more SCSI peripherals than a single bus will accommodate. And while it may be heresy to say so here, I've never felt particularly warm and fuzzy about the gigantic, incredibly heavy Q900/950 form factor. I had an AWS95 (Q950 with extra PDS card) once, and while it was fun, I sold it because it was so darned big and I couldn't get it to boot from the fast SCSI PDS card.
You know, that's a really good question. The PPC Macs based on the 630 architecture were terrible, but the '040-based 630 platform itself is really nice. The ability to use IDE drives is very nice - BUT, you're limited to 4MB of mobo RAM and only one SIMM socket. Some models have two SIMM sockets (LEM says the Performa 631 does), but these came with 'LC040 CPUs. So you'd have to go get a full '040 CPU, and even then you might or might not be able to use SIMMs larger than 32MB. So a Q650 or 800 will easily allow you to install more than 68MB RAM without spending a lot of money or 64MB or 128MB SIMMs, and without having to spend any money on a CPU upgrade.
I don't know if both cards will work stacked like that, but I can say that I had a Performa 640CD/DOS once, and it was the most fun I've had using Windows on a 68k or PPC Mac - very cool!
Others may have a different view, but my opinion is, if you really want to use *nix on this machine, go with the 650/800 board. The IDE-based Macs like the 630 are an absolute nightmare for *nix IMHO. I've never successfully installed it on any of them. And the 650/800 have the extra bonus of being supported by A/UX.
Best,
Matt