When I look in /proc/cpuinfo, one of the lines tells me I have 6144 KB of cache. Is this L1 cache, L2 cache, or the sum of both of them?
In any case, how do I find out how much L1 and L2 cache my system has?
That is L2 Cache. 6144KB is actually 6MB. You can easily check it on the Wiki or on CPU manufacturer's page.