I run a 17" CRT display at 1024 x 768. (Sometimes at 1152 x 864).
I believe a 15" flat panel has approx the same screen area as a 17" CRT.
What resolution would either a 15" (or a 17") flat planel display need
to run at to get approximately the same dots per inch as I currently
get?
I think a normal screen has a 4:3 aspect ratio. Using Pythagoras,
("the sum of the square of the sides...") half the screen is a
345 right triangle. The hypotenuse is 5, the sides are 4 and 3.
Using ratios, you can get the sides of the screen as 13.8" and 10.2".
Using your 1024 and 768 res., this is about 75DPI on each axis.
(Not that any of this matters of course.)
 
17*3  17 diagonal 768  About 75DPI on each axis
  345 triangle  
5  10.2 
+ +
17*4 1024 / 13.8

5
Again, if I understand your premise properly, I can use
ratios of 17/15, to figure out how many more pixels would
be needed. This is because both the 15" and 17" screen are
345 triangles, and the sides of the triangle are a function
of the diagonal dimension in both cases. And strangely enough,
the number of pixels is roughly the next standard screen
resolution, so 1024 on one screen, needs 1152 on the other.
1152 on the smaller screen, would need 1280 pixels on the
larger screen.
17/15 * 1024 = 1160, so 1152 on the 17 would be the same
as 1024 on the 15.
17/15 * 1152 = 1305, so 1280 on the 17 would be the same
as 1152 on the 15.
In the above, 1160 is roughly the same as 1152, and that is
why I selected 1152. in the bottom one, 1305 is very close
to 1280 standard value.
Does that seem reasonable ?
When I bought my LCD, I went to the store and extensively
tested it (much to the annoyance of the store staff). I spent
a couple weeks, and multiple visits to different stores, before
I made my purchase. The hard part, is getting the damn movie off
the monitors, and getting the Windows desktop and a Notepad up
on the screen, to test the quality of text rendering. The store
that let me play around the most, got the sale.
An instore test is a valuable part of the selection process,
because you'd be surprised how bad some of the cheap $300
screens are for text.
Paul