nem, i did not mean to confuse you. if you only get a 15" tft you are struck to 1024x768 - and 17" tft mostly have that resolution too, but there are ones that are at least 1280x1024 or higher. a 15" tft is about as big (from useable place) like a 17" crt, a 18" tft about as big as a 21" crt. and there are analog tft, that get a normal analog signal - this you dont want to have, you better get a tft with digital (dvi) input - but you video card needs to support dvi out. most modern graphics cards that have 2 monitor support have one dvi and one analog output.

and one advice, never ever buy a tft without seeing if you like the image and that there are no pixel defect (you need to let you show a white and a black picture to see this).