I think Cleo may be a little optimistic on that speed estimate with GPRS. With GSM currently you'll get performance very similar to a 56K modem if you have a good clear signal. If you see 128K, it's very likely with compression.
EDGE is much faster but not all that widely available and carries a price premium.
In either case, make sure you get an "unlimited" data plan and make sure the time you're using it isn't charged against your minutes. As you well know, you can burn thru a couple of hours and several megabytes of data in a hurry just tending to email.
Another alternative is an old college trick with those public computer rooms. Learn how to make the whole room go down whenever you need to get online....most people will just get up and leave instead of wait for it to come back up. All ya gotta do is find the right wire to temporarily unplug.
