Probably not even 8GB RAM as a minimum requirement. That would mean the majority of game buyers with (4GB or less) would not be able to buy the game. Thats not the way to go about trying to make a profit.
I think all the cash is always used. If data needed is not in cache then it is pulled from RAM. The hard part is to predict what will be needed next and to load it in cache before it is needed. It makes sense to load as much data as possible in the cache to increase chance of right data being in cache so it should be always full unless OS and software needs less memory then cahce which is possible with DOS but not Windows.
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).
Hmm, I did some bad math above. 12mb/4 != 4mb each. Core2 quad combines 2 wolfdale cores in one package. Each wolfdale core has 6mb L2 shared between 2 physical processors. So 6x2mb = 12mb. This doesn't change what I said above though, just thought about it a minute ago and realized it didn't make sense so I looked it up.