RE: Minecraft (1.3 Discussion)
It should be fine for like 6-8 folks, but you'll have a hard time playing and hosting on the same machine.
It also depends quite a lot on your upload speed.
I wouldn't leave it running for a long time though. I've read lots of reports where laptops overheat and die when hosting a server like that. Running only in the evenings should be fine, just don't let it sit there 24/7.
If it were me, I'd use a different machine, or if you really want, you could rent one for a monthly fee. But again, just me.