I recently upgraded my monitor to a 21" display. Is there a "recommended" resolution for a 21"? Should I pick the largest I can finde with the highest hz my video card delivers? For instance is 85hz preferable to 75hz?

The higher the screen refresh rate the better on your eyes, but if you go too high, it can blow your card. Stay within the recommended guidelines of your card.

Well, back when the Mac was first doing the desktop publishing thing, resolutions were critical as far as getting printed output to match what was laid out on screen. 1 to 1 ratios for this purpose (assuming a screen resolution of 72 dots per inch - Mac OS standard, not monitor resolution) were 640 X 480 = 15 inch, 832 X 624 = 16 inch, 1024 X 768 = 20 inch and 1280 X 960 = 21 inch. Nowadays, with screens and video cards pushing such high resolutions in smaller monitors, most people will opt for what looks best to their eyes.

As far as what to get, find a monitor size that will fit your needs and your wallet and then check ratings from various sites or magazines to see what is best in your size of choice (of course, it wouldn't hurt to eyeball a few of them first to see if they're what you'd like to look at every time you use your Mac).

I just picked up a refurbed 19" ViewSonic PF790 from Ubid for about a $100 less than the lowest priced new one I could find. With its Sony-style flat-square tube, it's very sharp and focused even at the 1280 X 1024 @ 75hz res. I'm running. It does suffer from a bit of blurring in the corners but that's normal. Much better quality than the 17" version that they had to return from work due to bad screen clouding.