If you've just upgraded or replaced your RAM, then the first thing you should do is test it.
Bad RAM can be a huge problem when it comes to system stability and corruption of data. It's a near silent killer, with most users more likely to believe that there's something wrong with their software than their hardware, or that a file "just went bad".
Thus, the very first thing you should do upon inserting new RAM into your system (or, indeed, when you buy a whole new system) is check the memory.
The easiest way to do this for Windows and Linux users is with Memtest86+ — a free program that comes on a bootable DVD or USB key. Download Memtest86+, create the bootable DVD or USB key, set your computer to boot from optical or USB through the BIOS/UEFI and then restart and let Memtest86+ complete at least one cycle.
It'll take time, depending on how much RAM you have, and you should let it complete at least one cycle (note that it will run endlessly if you leave it alone). If you have more than one stick of RAM in your computer, Memtest86+ won't be able to tell you which one is the bad one if it finds problems — so, ultimately, it may be best to remove your additional RAM and test one stick at a time.
For OS X users, you have another choice: Apple's Hardware Tests. Check out Topher Kessler's write up here for more.