I'd take this question to a couple of places on
http://www.dpreview.com - both the Pro forum to see how people are shooting weddings with 6 meg cameras and the Canon 10D forum to ask specific lens questions.
If you need to shoot groups of people and print those images large you may have to work to produce a quality product with a 6 meg camera. If the need is rare then you can shoot multiple frames and stitch together.
Six meg files will make great large prints if fine detail isn't important, and with portraits fine detail generally isn't wanted. (Who wants blow-ups of skin pores?)
I've been shooting an IS camera for the last few years and suspect that you will get more value from that feature than you would from a super sharp lens.