Here's a brief rundown of the 5xx series for you:
510 = basic
530 = 510 + home base compatible & included
560 = 530 + lighthouses and onboard scheduling (wireless scheduling compatible but you'd need the wireless control center)
570 = 560 + wireless control center included
There are a few other models floating around (like the 535 & 550), but those are the main ones
iRobot sells in the 500-series. Lighthouses may be nice, but aren't strictly necessary at your house size. It's up to you how much you care about scheduling.
I'll let someone else comment on the pet models, as I don't have any (well, except a goldfish, but he's messy in a non-
Roomba-fixable way

).
If you have a fairly simple house with no rug tassles or without many bumps, you can get away with one of the 400-series. But the 500 are much better at navigating those things, so if you have anything that makes your floor less than simple (especially rug tassels!), you'd be much happier with the 500 series.
Anyone else think maybe we should sticky a thread like this? As this question comes up a lot.