I think this might of been brought up before but why not a canon 50mm f/1.4 ?
I can shoot this wide open with no flash handheld. for portrait and low light work, you really can't beat the sharpness of it.
I know people who use the 18-200 however with a range taht large you won't get the sharp pictures you might be looking for.
Here is a pic with the 50mm f/1.4
i actually looked at one over the weekend. They didnt have one for the canon at the time, but I did like it. I will probably get one when their shipment comes in friday or next monday.
Sorry about the big image