I can understand not being able to make your own sig. I heard someone mention sidetrack, its good, but needs a lot further development, plus your getting an imac, no track pad, so its unneccesary. Id also like to recmomend konfabulator, its really cool, until tiger comes out, if you wanna spend another $130 on it, itll include the new dashboard feature.
I also think you might like apps such as candybar, pixadex, and shapeshifter, and cleardock.
also, try adium or fire fore chat, adium cant transfer files, so use ichat just for xfers.
and carbon copy cloner for backing up your stuff.