I don't think this is much of a problem.
For example, mastercard has maximum capacity of 140 million transactions per hour. Which makes 38.888 transactions per second.
38.888 transactions per second * 250 bytes = 9722000 bytes = ~77,77Mbit. (Though on average, Mastercard does only 22 billion transactions a year, which makes about 2,5 million a hour, and ~700 a second, so there are probably only spikes with high number of transactions, and the rest is much lower)
So anybody with 100Mbit broadband/fiber will be fine. Just buy a powerful 6-or-more cores computer, and you probably will be able to easily calculate everything without much strain.
But by the time Bitcoin will be so popular to process that many transactions, internet connections around the world will be easily 100-200% faster than currently, so no problem there. Also, you can always buy a decidated hosting connected to 1Gbit pipe (which is less expensive that you would imagine), and You're cool.