CPU Cache Size

because competition getting tough bw amd and intel just like ati and nvida till now best gaming processor is amd fx 8150 and i5 2500k and i7 2600 k these till now

the cache doesn't really matter in dual core series which have 2mb or 4mb cache but these cache are in good amount for those cpu's..
Cache is just like A RAM but cache are indeed extremely fast memory than ram. The manufacturer uses lower cache in the development of newer cpu's if they uses higher cache then it will be just a waste an example of core 2 quad q9650 it has 12mb of cache however i3 2100 uses only 3mb of cache but 100mhz more clock speed, now if we compare those two cpu's the q9650 is beaten by i3 2100 in some benchs, even when there is a huge difference 12mb beats by 3mb in some way.
www.anandtech.com/bench/Product/289?vs=49

on the other hand q9650 has 4 cores where as i3 uses 2 cores

it becAUSE i3 have better technology its have sandy bridge 30 nm which make faster and core 2quad is 45nm chip ddr2 support i 3 start from ddr3 so its perform well

Keep in mind the following guidelines:
- cache size and latency are proportional (inceasing cache increases latencies)
- cache size and manufacturing costs are proportional (a smaller cache can make a whole waffer contain more CPUs, thus more units at a certain price)
- cache size and power consumption are ...well I don't know if proportional or exponential...no data to back it up.

So when they test what cache size to use, they consider whole power usage, chip size and speed benefit. Because going from 1->2MB will bring more performance than going 2->4MB. So while 4MB will be best performer, the other factors may keep it at 2MB (for power, maybe an increased clock will get better performance overall).

As for Core2Duo and i3 (and any i's for that matter) the comparison of only cache cannot be done. i3 has an onboard memory controller, thus reducing RAM-cache latency. Also it has improvements in the execution engine.

PS: Core2quad/duo can use DDR3, as the memory controller is integrated in the MB, not CPU (as i3).

that what i need to say new machine is always ahead then old one it will make by experience of old one like c2d old i3 new one

Thanks Matthew, that's good info. It confirms what I was thinking - that the cache cost (in acreage, energy and cash) goes up faster than the benefits, so caches are self-limiting. The background is I am writing a benchmark utility and I don't want to have to future proof it against theoretical 64MB caches.