Unless you're wanting something special, I'd wait for a Petco $1 per gallon sale. I think they're usually Aqueons. If your local carries 55's, they'll be 1/2 off instead of $55, still a good deal, but 40's should be $40. You have to buy your own glass top.
My store looks like it might be happening now. It comes out of their corporate office, so I'd call your local store to see what sizes they have available.