Just read 'em the Henry V St. Crispian speech over vent!
But Seriously, with a roster of essentially 32 you will have a hard time filling raids unless you are a hardcore raiding guild. If nothing else, the weekly events in people's lives will run you under 25 often enough to be a hassle. We're a mandatory attendance (if you're online) raiding guild and we need to run a roster around 40 people to keep raids full and have enough on the waitlist to keep it rolling to the end of the night, given people's comings and goings.
The fact that you define yourselves to be casual raiding (I assume that people can decline to raid without penalty?) pretty much demands that you recruit until you're filling raids to satisfy desires. This can be a real sticking point for small "friendly guild" types. It also will potentially create tension when teams A and B are finally both rolling Karazhan cleanly and other people suddenly develop a keen interest in raiding. Fair-weather raiders are fun.
The next best thing is to enter an alliance with another guild or two in a similar position. I personally would never give myself that headache again, but others have good success with it.