DDR3 1866/1800/1600/1333/1066: which one is sufficient at stock speeds?
Honestly, I'm still using DDR2 memory, and I find them more than adequate; so I'm pretty sure that any of those choices will be fine for you. But those numbers refer to speed, so the higher they are the faster your memory will be. Keep in mind though that your motherboard can only go as fast as your motherboard's specs; so if you buy 1866 RAM and put it in your motherboard which can only handle 1066, then that's how fast your RAM will go.
CAS latency: what is that? I guess that lower is better?
Yes, the lower the better.
Timing: what do those numbers mean? 7-7-7-24, 9-9-9-24, 8-8-8-28?
The first number in the timing sequence is the CAS latency. The other numbers IIRC refer to other latencies in the RAM, once again lower is better.
Are there brands to avoid?
I recommend OCZ (I have had two sets of their Reaper Edition) and mushkin RAM.