I did a quick Google search on the topic, and discovered an old Visa press release that stated that during the peak hour for the year of 2005 (Dec 23rd) Visa processed an average of 6,363 transactions per second, or just shy of 3.82 million transactions within a ten minute span.
That's a lot. Bitcoin won't allow blocks over 1MB, so assuming a (rather small) average transaction size of 216 bytes, Bitcoin can only handle 4,629 transactions per 10 minutes.
That hard limit is going to have to be bumped up if/when Bitcoin takes off. 463 transactions per minute wouldn't be anywhere near viable for a worldwide online currency. We, at least, need to be able to handle the number of average transactions per minute that paypal claims they currently average.