Not that wide though for DX users..?
Personally I would keep the 18-55 and save your money - it's a great lens, especially the VR version. I had one and regret selling it.
I don't think you'll notice a huge difference with the 18-70 in image quality. The only advantage I can see with the 18-70 is slightly better build quality and an extra 15mm on the long end, but you'll lose the VR with the 18-70.