Originally Posted by
Saracen
Not according to Adobe .... though it's a moving target, depends where you're at with other things, depends what your work profile (file sizes) are and as always, it's a blend of issues.
But, even assuming you're on a 64-bit OS, there's limited gains from taking memory up much above 8GB. Small gains, maybe, but according to Adobe's tests, very small. So .... multiple cores gives benefits in some functions but isn't really that beneficial, certainly once you get above 2. You'll gain rewards from increasing total disc I/O with multiple drives, and from RAID (hardware controllers and performance oriented levels, like 0, 0+1, etc).
Overall PS performance is obviously going to be constrained by the weakest link in this chain, so it's apparent that you need to be somewhere near the optimum configuration, within the bounds of budget. You'd be better served by increasing a single disc to two or three as opposed to increasing memory from 8GB to 16GB, but what if you've already got three "drives", with the scratch disk on a RAID 0? Then what do you look at?
Increasing memory to 6GB, or 8GB, is clearly a pretty cheap option these days, and for a heavy Photoshop user is pretty much a no-brainer provided you're using a 64-bit OS, and that is pretty much a no-brainer providing you can get drivers for essential hardware, etc.
But, according to Scott Byers (Principal Scientist at Adobe) Photoshop is a memory-bandwidth bound app. Part of the reason for that is that some people use VERY large files, and they aren't going to fit in memory. And even if they do, as soon as you start working on them, you're going to be creating layers etc and you'll rapidly need several times the size of the file which, if you're working with GB-sized files, is a serious demand. But also, and perhaps more importantly, when you're applying filters and effects, the critical point is how fast you can get data from main memory to cache for the CPUs to get hold of it, and that is where bandwidth hasn't been keeping up with CPU development, etc, over the years.
And i7 shows some signs of changing that somewhat, because of those architectural changes I mentioned.
In other words, going back to the OPs question ..... is i7 worth it over the Q6600? I come back to what I said - depends what you're doing, but bandwidth-limited apps like Photoshop might be the justification. After all, if increasing memory was going to add benefits, given the relative cost of an extra 4GB, he'd add that in either way. If spending the extra £250-£300, or more, on i7 is an option, the extra on memory didn't ought to be a problem.