Hi, Doug...
I find running *any* guest under a 32-bit host is less than enjoyable.
I have VB running under Win7 Home edition (32 bit on my Lenovo T510), and under Win7 Pro (64 bit on my Lenovo L530). Again, I don't use them much, but I really don't see much difference.
It's all in what you run.
Our tax prep software from CCH (ProSystem fx), while being very well written and well behaved (for a Windows app - LOL - and they started on OS/2, back in the day!), is a resource hog. I feed the VM 3.5GB RAM, and unless I reboot the VM every night at 2am, it is dog slow by morning, running on a "dedicated" host machine (I have three other - slimmer- VMs running on that same box). In 1GB or 2GB, I could not effectively run the application (I've tried). It gets too slow (wait for screen updates, wait for pages in the app to draw, switching to other apps running in the same VM is slow, etc.). In addition, my brother and I share the VM, like a poor man's terminal server (yes, I tweaked the install to allow multiple simultaneous RDP connections). But even when only one of us is logged in at one time, it gets cramped really fast in less than 4GB. The app itself is 32-bit only, so switching to 64-bit Windy isn't a real answer, either (and as you so rightly point out, 64-bit guest requires a 64-bit host, anyway).
This will go away to some extent next tax season, as I'm switching back to a Citrix solution (but virtualized on 64-bit Linux (I always get a kick out of that; it's kind of like Bill Cosby's take on what his parents had to say about baby sitters in "The Chickenheart": "What? Let some stranger stay home with my kid? I'd just as soon leave him by himself!" - and that's how I feel about running Windows on bare metal - LOL!).
Again, it depends upon what apps you need to run in the VM. Heck, I can't run XP on *any* machine with 1GB and get any kind of performance.
Finally, as for physical memory, RAM is cheap. I just built a really FAT Windows 7 box for my nephew, who was complaining about the poor performance of Adobe Creative Suite on his T61 with 8GB. The new machine has 32GB RAM under an i7 CPU and a dedicated nVidia graphics processor (egad). The RAM was the least of the component cost (and BTW, 32GB = 2x16; there are still two slots left - LOL!).
Cheers/2