OP:
I'd like to offer a suggestion that might be of use to you with your current iMac.
Have you considered trying one of the 27" "mid-resolution" displays?
By "mid-rez", I mean a 24" display with a native resolution of 1920x1080.
This is roughly the equivalent of taking a 24" display and "enlarging the pixels" to fit a 27" form factor.
The result is that the pixels are larger.
Younger folks with good eyesight will look at one of these (even at 1920x1080) and blurt out, "that's too grainy for me!"
But for us older folks, or for people with not-so-good eyesight, it's MUCH easier to see and read.
I'm using a Viewsonic 2770, like this:
http://www.amazon.com/ViewSonic-VX2...id=1414942989&sr=8-1&keywords=viewsonic+2770\
(no financial interest)
... but there are numerous other IPS displays using the same form factor out there.
A good place to check for them is by viewing the guide on this page:
http://www.pchardwarehelp.com/guides/s-ips-lcd-list.php
Cost will run in the $250+/- range.
Attaching one of these to your existing iMac might offer a usable solution...