For most situations, I use Photobox
http://www.photobox.co.uk/
You do not need to resize when sending files to them, as Alex say, you're best off sending the full size, max resolution image.
For really, really critical work i use proam imaging
http://www.proamimaging.com/
These people do not have an online upload service, you have to send them the files on CD, and you have to resize and follow their image preparation instructions very carefully. It sounds daunting, but isn't really, it just takes a bit more time. The service is slower than photobox, the effort you have to put in is greater, but the results are really very good, especially their mono prints.
Duncan