we start 2 rb and 2 wr...standard scoring, 6 pt TD rushing or receiving and bonus points for 75 yds and beyond (not ppr).

owner is offering Welker and choice of Benson or Fred Taylor for an upgrade at RB.

My RBs are Edge, LenDale, and Gore
My WRs are TO, Galloway, Muhammad, and Crayton

I'd love to get Welker...which RB do I move, and which RB do I ask for in return?

I would do this and give up LenDale, personally. Not sure which RB to get back as neither are appealing choices, but I would lean toward Fred Taylor.

He wants Edge--I said no.

so you feel that the upgrade from Galloway to Welker is worth the downgrade of LenDale to Taylor?

(I actually prefer Benson myself, since he gets whatever goaline carries are there)

I think Welker is a nice upgrade for you, and should be more than enough to cover any downgrade at RB. Benson is closer in scoring to MJD than Taylor is to Benson, so that's a no-brainer. White is at the same level as MJD also, so you're only looking at about 14 points total from them to Benson, even as bad as Benson appears to be. Taylor only has value if MJD goes down.

I wouldn't give up James, but I would do it for either of the other two. You only need two backs, and you need help at WR.