Good advice all around. So long as you conduct yourself well, and they aren't a bunch of children, there's no reason you can't leave with your dignity and reputation intact. I would second the notion of finishing out gigs they have booked if they want you too, but definitely fix a solid date for your complete departure. Helping them find a replacement might be nice, but it's probably better to let them choose your replacement. I tried the stick around and help them find a replacement thing, and it took them over 6 months to let me go, they still hadn't found a bassist, and they broke up 3 shows later! Give them a month (maybe 2) to use your services as part of the band, and beyond that, if they need your services, you're a hired gun.