put it this way, how much do you need? Do you really need 8 gigs on one card? With my EOS 1D mkII's I can fit somewhere between 150 and 250 photos on a single 1 gig card at the high quality (obviously less with RAW, but I rarely shoot raw). With your camera (Rebel, correct?) you will be able to fit nearly 1,000 pics on a 4 gig card ...
the reason I point this out is flash memory will wear out, it is designed for x-number of write cycles, so unless you are filling the whole card, the 100 to 500 shots you might shoot on a heavy day will only fill up parts of the card while much of the card is not ever being used.
In addition, cards do fail. Not often, but at the worst possible times. Having a second or third card handy is definitely an advantage - so I would say go for a couple of smaller cards.
If you find yourself routinely filling up two 4 gig cards, then buy an 8 gig - but I really can't picture that happening often.