Can anyone quantify the benefit to be gained by removing icons from the desktop? (In terms of available memory or processor performance.)

It probably could be quantified, but I doubt it'll come down to 43 icons is ok, but 44 will cause you trouble.

In general, the fewer resources you use, the more memory you'll have for running apps and the less swapping (memory to/from page file) will occur.

So keep desktop clean, clear out the Dock of little used apps, don't use dashboard, etc.

All generally common sense things, really.

Oh, and moving things from the Desktop to a folder, and then having that folder open, is just as bad, if not worse.

well, it seems to use between 20-50k per icon (see kBytes column in Quartz debug). So if you have 100 icons, you'd save at most 5 megabytes of memory. woohoo, err that's not much.
Now we'd need to compare the refresh rate (or time spent drawing the desktop) when say a window is moved on top of the desktop when there are 1 or 100 icons.
I bet you'll only see a few % difference in CPU usage in Activity Monitor. At most.
Most of it is saved in quartz's buffers anyway, only the revealed regions need to be redrawn.
So you'd have to count : comparing 100 rects (not much), then drawing n icons (not much (remember the icons are already loaded (ok they may be swapped))).

Really, (I must say I removed all icons after reading the hint ^_^), it feels faster, but it's probably just tidier !