Can someone tell me how much faster an E6300 would be compared to my present P4D 920?
On average, an E6300 would be about 1.3x faster than a D920 - probably
roughly equivalent to a D960.
The E6300 is definitely the best value out of the Core2Duo CPUs, although the E6400 isn't far behind in the price/performance stakes.
It's just whether you feel £120odd is worth it for a c.30% CPU performance gain (assuming your current motherboard can take a Core2Duo CPU).
You could sell the 920 though I suppose - if you stuck it on Ebay, you'd probably get someone to buy it for £100
On a side note, it seems that from the benchmark figures on Tomshardware, the performance of the Core2Duo chips seems to follow the clock speed pretty accurately - there doesn't seem to be any performance jump when you go from the 2MB cache Allendale cored E6400 (2.13GHz) to the 4MB cache Conroe cored E6600 (2.4GHz), which isn't attributable solely to the increased clock speed.
For example the H264 test
E6400 (2134MHz) - 285
E6600 (2400MHz) - 254
That 254 secs for the E6600, is what you'd expect to get if you clocked the E6400 at 2400MHz (285 x 2134/2400=253.4).
It's pretty much the same story on many of the other tests too.
The test systems are identical apart from the CPU.
I would have expected the E6600 to have a c.12.5% performance boost over the E6400 just from it's higher clock speed - but I also expected to see a bigger jump than that due to the increased cache.
Are the tests flawed, or are current systems/apps unable to take advantage of the larger cache size? Have results for some models been extrapolated based on clock speeds? Hmmmmm.......
Anyway, one thing is certain - the E6300 (at stock speed) is around 3x faster than an AthlonXP 2400+ when encoding in x264