Would a 18x8" rim fit on a xB? If so what would be the correct offset and tire size?

They'll fit but depending on the offset you get (according to what I've read) you'll get some rubbing. Here's some links on the topic:

http://www.scionlife.com/forums/viewtopic.php?t=3921&highlight=18x8

http://www.scionlife.com/forums/viewtopic.php?t=4123&highlight=18x8

http://www.scionlife.com/forums/viewtopic.php?t=348&highlight=18x8

http://www.scionlife.com/forums/viewtopic.php?t=3922&highlight=18x8

Sure, an 18x8 will fit... do you want to put it on the front or rear? Front? +30mm to +32mm offset. Rear? +25mm to +30mm offset.
Tires... 215/40-18 will work, as will 215/35-18.

Sure, an 18x8 will fit... do you want to put it on the front or rear? Front? +30mm to +32mm offset. Rear? +25mm to +30mm offset.
Tires... 215/40-18 will work, as will 215/35-18.
Will they rub with the Tanabe springs (2.2-front - 2.0 rear)?

Sure, an 18x8 will fit... do you want to put it on the front or rear? Front? +30mm to +32mm offset. Rear? +25mm to +30mm offset.
Tires... 215/40-18 will work, as will 215/35-18.will the 215/40/18 rub w a 2in drop?

it shouldn't have any rubbing issues 215/35/19" fits with little problems and 215/40/18 has a shorter overall sidewall, so you should be all good

i'm going with 215/40/18 on a 18x7.5" wheel +42 offset, running a 5mm spacer in the back (maybe a 5mm in front and 10 or 15mm in the back, but i'll see what it looks like with the 5mm in back first)

i should have them on by next weekend, i'll post up some pictures as soon as i can

oh ya, spfr jon has 18x8" wheels +45 offset, and they fit on his car - he has to run a 5mm spacer in the back and he had to do a little trimming on his ride, but it was only minor and he's driven cross country with those wheels, so even the higher offsets will work, but it's going to be more drama - try and get the lower offsets if possible!