I wouldn't sweat the 490 too much, especially if you came in to the test center in less than perfect mental condition. When I was prepping for the exam and taking practice CATs, I had similar experience - got in an argument, was definitely still agitated when I sat down to take the test and scored abysmally low. It happens to everyone.
Re: the Kaplan diagnostic exam. If I remember correctly, there is a paper exam in the book and there is another on Kaplan's website (to which you should have access since you purchased the book w/ CD).
For study materials, you might want to read through the verbal section of your Princeton Review. I haven't been around here long enough to see whether other forumers recommend it, but I found it extremely useful. The strategies and tips they offer were invaluable; especially on the RC.
For the math, I thought PR did a great job explaining the general strategies, but Kaplan was much better at the actual equations. And thier practice problems seemed harder than the actual GMAT questions so, if you can knock out the Kaplan questions, you'll breeze through ETS's.
Re: one month to prepare - it can certainly be done!
I took the exam last October and had perhaps 5 weeks to prepare. And, in addition to studying for the test, I had a full load of classes and worked full time. If I can go from a 430 (my first PowerPrep CAT score) to a 710 on the actual test in 5 weeks, you should definitely be able to reach your goal!
Good luck!