If you really can't wait to see what's going to happen, then I'd take the 1.8ghz machine if it's a dual processor tower. Otherwise, I'd just stick it out another month to see what comes down the pipe at WWDC. If the offers aren't time-limited (as such things sadly are, most often 🙄 ), then you can't lose.
Worst case, you get the machine anyways. Best case, you might see something even better around the $1,700 price point. How can you lose, assuming that the offer doesn't end before then?