ok then its easy to say get a 22" monitor you'll be happy... but unfortunatelly not. If you don't have 2 x 8800GTX SLi don't dream about it. To make it work Lock-On 1680*1050 which is native resolution of 22" monitors with full details, you need really fast computer otherwise you can't get more than 35-40FPS normally. Btw if you get many planes around of your plane in game, your FPS can decrease to 10FPS even :)
Usullay I get 70-80FPS with my spec but its because of I overclocked my computer and I pushed the limits. If I were you, I would like to get 20" 4:3 monitor certain and run Lock-On with 1280*1024 native resolution. Don't forget experience is talking here.