I used 2x6 for my 125g stand with L's in the corner and additional 2x6 in center front and back. Of course this was a 4.5' long tank so I needed to overkill it a bit since it was a taller tank. You can see build picks in my only profile album I believe.
Anyhow, like post #3 said, there are tons of plans in the diy stand section. Just research them.