Well considering you're currently running a p4 and only one brand of processor has 12mb of cache, I assume you're thinking about upgrading to a core2 quad. I'd say go with i5/i7. The architecture used in those processors is superior to the architecture in the core2 generation. 12mb L2 cache is misleading because each physical processor can only see 4mb of it each. i7/i5 is more efficient because even though there is only 256k L2 dedicated per core, there is 8mb shared L3 cache between all the cores so when cores are inactive, the ones being used can make use of 8mb of cache. For instance, if you have only one core running, it has access to all 8mb of cache, 2 cores will share the 8mb and so on. This means there isn't as much inactive die space. Purely from a cache standpoint, I understand that this argument isn't very decisive because with all cores active there is less available cache per core compared to the high-end core2 quad processors. However, i5/i7 have a lot of perks that boost performance which I won't get into (there's plenty of reviews out there).
For reference, here's a benchmark suite with the cheapest i5 (i5 750) vs the most expensive c2q (q9650):
http://www.anandtech.com/bench/default.aspx?p=109&p2=49
i5 wins all but a few benchmarks and is $130 cheaper (on newegg).
Anyway, hope this helps...