The LPC21XX have 5v tolerant I/O so you can talk to a 5v part with no problems. Since these have to be pulled up you can pull them to what ever the device(s) you need to talk to run at. As for the value of the pull up resistor usally 10k will work fine, if you have alot of devices on the bus you might want to go with a little stronger pullup (lower value resistor).