I'd like to add that I added 8gb Corsair ram and it also said 8gb ram @ 1333 when I checked about this mac. But soon after I started to get kernel panics so I did memtest and found one of the sticks defective. I put the original 4gb ram back in and no more kernel panics. I rma'd the Corsair.
Apple hardware test which is located on one of the DVD's that came with the machine, check apple's support site for instructions, it's pretty straightforward.
A lot of times the Apple HW test will not detect a defect in problem RAM b/c it's not defective, it just doesn't meet Apple's high standard. But if you used your MBP for a while w/ the OEM RAM and had no issues then put in your 3rd party RAM and started getting odd behavior -- random freezes, graphics breaking up, kernel panics, etc. then 100% guarantee it's the 3rd party RAM. The only general purpose RAM, i.e., not specifically labeled for Macs, I've ever found to be 100% consistent is Crucial. I have a good, but not perfect, experience with PNY. Kingston ValuRAM is 50/50.