My E8400 at stock is appreciably quicker than my E2180 was at 3.2.
The cache does make a difference, as you say, with Super PI. But, I've done some tests with the Crysis benchmark and although it hasn't made much difference to the average or max framerates, the minimum framerates are approx 8 to 10fps higher. Playing the game, there certainly seemed to be less stuttering moments. The only unknown in all this is how much difference the motherboard has made, as I changed to this budget P31 to ensure Wolfdale compatability. The old 650i board was no slouch though, and still had 16x pcie lanes, and looking through my old 3dmark06 scores, they were comparable to the new setup in graphics performance. But, that aside, I'm assuming the extra performance is down to the cache, but can't say for sure.
I also do a lot of batch processing of 4mb JPG's in Paintshop Pro X2, and again although the E2180 did a sterling job, the E8400 at stock is about 20% faster than the E2180 at 3.2.
A little more scientific test is the Custom PC Media Benchmark, which is a video encoding, photo editing and multi tasking test. An E2140 I had running at 3Ghz scored 932 points, and an E4400 I had at 3.4Ghz scored 1028. The E8400 at 3Ghz scores 1197, and at 3.8Ghz it scores 1395 points. I did have a G0 Q6600 CPU for a time, but it wouldn't overclock in my N650i board, but at stock that scored 1068.
Overall then, it would appear to be about 22% faster than a E2***/E4*** chip at the same speed, in that benchmark. At stock, my E8400 is 12% faster than the Q6600 at stock, although probably to be expected given the extra clock speed of the Wolfdale.
The above are just my ramblings of course, so nobody get upset!
My thoughts at the moment are, ok, it's faster at photo editing than any other chip I've had, which is probably the main use for my CPU other than gaming. But, lets be honest, my life hasn't changed because of the few minutes I've saved processing a few photos!!
But, I've got a pc that now it's overclocked is about 30% faster at most things than anything I've had before, so for me that's worth the cash.
To be fair, if the quad had overclocked on my old board, then I probably wouldn't have tried this E8400, but I enjoy trying new stuff, so why not?!