If your staying in polys, move the verts that are along the creased line back away from the midle in the x direction. Move them some what away from there. You will just have to expirament to get the feal of how to get what you want. But you should have good results from the start. The key is to leave at least 1 vertix on the midle line or very close to it.. the poly mirror geo will brige the gaps nice but kepp the 2 mirrored right ware they should be, becouse of the one vertex forcing the mirrored side to the right spot. but the mirror poly ( with attach terned on .. LOL ) will bridge the gap on the rest of them.

The reson the seam is there is becouse even thought it might look like there is just one edge there when you mirror. when you smoth it dosn't treat it like that.. it treats it like there are 2 edge there.. and it puts the extra geo that makes it smoth, inbetween thoughs spots, forming a edge..

I'm probly explaining porly, I could provide pic's if needed..