These CPUs are compatible with the system:
Yes, you can upgrade it. I upgraded two of them to 16gb of memory. And one of them to an i5-3550S CPU. Both took the memory upgrade just fine. The one with the i5 is likewise working nicely. Both saw improved performance.
I'm going to try upgrading the other one to an i7-3770S later this week.
These are the two RAM upgrades I used (one was a while ago, the other this past weekend):
Both passed extended burn-in testing with memtest86 (http://www.memtest86.com/)