L3 cache. It's a special memory that resides on the CPU. It takes time for the CPU to get data from the RAM, but L3 cache is VERY fast compared to RAM. The L3 holds data, so when the CPU needs it, it can get it very quickly. L3 is pretty important. Get either the Phenom II x4 965 (preferably) or FX4100. Don't get the Athlon.