I guess I just don't see the advantage of the i7 over the C2Q for the vast majority of apps.
I suppose that's true for the 'majority of users'. (Sorry I couldn't put it any other way

). But if you're a 'power user' who does heavy multitasking as well as encoding, i5/i7 is much more suitable platform, IMO.
The C2Q is great in a production environment running well threaded applications, especially those that do well on Intel architecture. However, when it comes to multitasking, the desktop C2Q's don't fare well against i5/i7 and PII. I did experience it first hand, and it looks like reviews around the web also confirm my findings.
I have moved to 8GB since Vista came out, and last year I got into VMs as well. My system is always using at least 3~4GB of RAM and when I start working/playing the memory usage can go up to 6GB. Sometimes, C2Q simply crapped out (pause, BSOD, or hang) and it'd mean that I lose work and progress in games.
The blame is probably on the chipset/board rather than the CPU, but they're still part of the ecosystem. (FB-DIMMs are out of question on desktops, obviously) And i5/i7 removes such bottleneck.
Again, I don't expect this scenario would apply to majority of users, but if you're running multiple heavy apps or are multitasking heavily, the benefit can not just be performance but can also be to crash or not to crash.
Well, as for Bulldozer and SandyBridge - they are way too unknown and far away to be a consideration. If you're happy with what you've got now, then no reason to upgrade. But if you're unhappy or see room for improvement, putting off upgrades waiting for BD/SB sounds silly.