Nigel -
Yes, it is possible to "upgrade" from an x64 version of Vista to any x64 version of 7.
"Conventional wisdom" dictating a clean install is best is probably true, for those unable or unwilling to learn some simple Windows housekeeping tasks.
I'm going to write this up soon for my site and I haven't told anyone because I know I'll instantly get labeled as nuts.. but I've been operating my main workstation (1366/i7, SSD's, 12gb of RAM, dual GPU video, etc, etc) on the same build that started life as a RC candidate of x64 Vista.. so that's about.. well.. from September 2006 to a few weeks ago. All upgrades, every SP beta and RC release throughout, and I've changed motherboards/CPU/RAM/Video cards so many times I've lost count. All without blue screens or other issues, at least none not immediately solved. I didn't set out to do this, but sometimes you start so far down a certain road and you can't help but wonder where it finishes.
Mine finished because of a timeout. I installed an RC release of Win7 SP1 and then accidently erased the backup files from the system, and then Microsoft timed out the RC.. and I was forced to rebuild a few weeks ago. Now that I'm rebuilt I'm getting the exact same benchmarks and performance scores..
So.. sure, you can upgrade and end up with just a reliable system as if it's a fresh install. There's really very little to it.
The second question.. will you benefit? Depends, are any of 7's new features jumping out at you as must needs? If not, you'd still benefit from 7's improved memory manager, but maybe not enough to be worth the upgrade. Personally, I'd be more concerned with upgrading that antique CPU of yours.. ;o)