There are several schools of thought.
One, of course, is the default: my PC is maxxed out on RAM, so I’ll just live with it until it’s time to upgrade. These days, of course, you can have a slightly different scenario — my OS is maxxed out on RAM… if you have Windows 32-bit and 4GB. Which may actually mean only about 3GB really available to play with — you get a small memory upgrade just going to 64-bit Windows.
There’s the common rule-of-thumb you’ll hear around here, 2GB per processor core. This works fairly well for Vegas, it’s overkill for some apps and insufficient for others. Photo work, for example, can be very, very memory intensive at times, but less processor intensive than video.
For HD in Vegas 9/10/11 on 64-bit Windows and a six core CPU, I have never run into a practical limit with 8GB of RAM. No paging, no other low memory issues. But some of that’s workflow… if I were using a different mix of apps all at once (say, Vegas and After Effects up at the same time), maybe I’d benefit from more RAM. I did upgrade to 16GB last summer, but that was for photo work (merging 25-50 18Mpixel photos to a panorama in RAW or 48-bit color… does eat memory).
For SD work, I was pretty happy with 1GB memory, though of course, that was like Vegas 6 or 7, so that’s less easy to compare to today’s Vegas.
You guys are right on about the Vegas “wake up” effect. When you de-focus Vegas and it’s otherwise not doing something, it releases locks on the assets you’re using. That’s intentional. If it didn’t do that, you wouldn’t be able to, say, jump to Photoshop to adjust a still or Forge to edit audio. When you re-focus Vegas, it’s going to grab those back, checking for changes in the process. In a big project with lots of assets, yeah, that’s going to take time. You’ll speed your project up (in general, but here specifically) moving some of those assets to separate hard drives.
-Dave