Don't know much about this Dell line, I did however have a few GX's come thru last week. No 280's but a couple of 240's.
The latch with the green tab is more than likely the expansion card lock and has nothing to do with opening the case.
There are 2 buttons, 1 on the top and 1 on the bottom ( or left and right depending on wether you use a tower or desktop configuration ) these are all that need to be depressed to open the case.
If you're have problems, try releasing one side then the other, worked for me on these systems.