By all appearances, GPUs have had a negligible impact on net processing power. We're still set to drop next difficulty change.http://mrx.im/pts.php
Also, I think you have the answer in your post: your 18 MH rig is making you less than $100/day on PTS, while you could almost certainly be making more than that mining, well, scrypt.
Why would I mine PTS with an AMD GPU? NVidia is a different story, but again, it does not appear to have affected much.
Anyway, if we believe in what's going on here, we should appreciate this buying opportunity, right?