Hi Pete,
The 17-40 has a good rep for image quality and handled well when I tested one (decided not to buy it though), and will carry over if you upgrade to a full frame body in the future, but doesn't do anything different to what your 17-85 does - it's sharper, but still only f/4 and has less range and no IS.
I went for the 17-55 f/2.8 instead - it's a bit faster and sharper than what you've already got, but like your 17-85 won't mount on an FF body. I'd look at one of these over the 17-40 if you want a straight upgrade to your 17-85 and you're not looking to move to FF anytime soon. The downside is it's quite big and heavy, plus they don't come up second hand very often so you might need to wait to find one for sub £500.
Hope that helps,
Nic