Yes, they can add your @mac.com screen name to their buddy list. However, if you want something shorter and not advertising that you have a Mac (or more an e-mail address to spam) you can get just a regular screen name at
http://my.screenname.aol.com <--what an ugly URL, but that's it.
For instance, my screen name is "itas0r". I also have
itasor@mac.com, but itas0r is much shorter and easier for people to remember.
As for the iSight, I would get it some place you can easily return it, because the AV part of iChat is known to do funny things, like not work for no reason. I just use my miniDV FW camera and it sometimes works with Windows users.