Before you go out and buy anything... let's try testing your RAM.
You can have 2 options:
1) Remove all RAM sticks, and boot with 1 stick at a time (trying differenet slots) to see if it's a particular stick or slot issue.
2) Download the ISO from Memtest86+ - Advanced Memory Diagnostic Tool
and burn it to a disc, boot off of it, and let it run for at least 4 hours (the longer the better). If it shows any errors at all, then you'll want to test each stick individually to find out which stick is the bad one.