There was an article recently comparing AMD's Athlon II quad(no L3 cache) vs the Phenom II quad(has the L3 cache). These are exactly the same chips except for the L3 cache difference. Tom's was find a difference when it came to gaming and some synthetic testing/benchmarks, but not a significant one.
My suggestion is if you want to save money, go to the CPU that doesn't have L3, as CPUs with an L3 cache aren't significantly better performers than their L3-less counterparts.
It just depends on what the computer will be used for; if it is just for browsing, playing 3D games, and playing media, a CPU without the L3 cache will do. If you do a lot of encoding, programming, video editing, etc, then the CPU with a L3 cache will help significantly.