I don't know about a bigger monitor but I might choose a different resolution though... Is resolution based off of monitor size or the type of resolutions your graphics card can be compatible with the monitor?
[Most] tests that I've seen show the 5770 CF >= 5870 AND you'll $AVE $$ ($100 w/5770 CF)
I assume that you "might" upgrade your monitor, adding another $160 to $300+ to your budget, if considering the HD 5770 CF <or> HD 5850 <or> HD 5770. Clearly, if you KEEP your monitor then a single HD 5770 would be more than good. The tests that you see, read or watch on Youtube typically have monitors 22"-24" with resolutions of 1680 x 1050 -> HD 1920 x 1080, or greater.
At minimum going from a 1440X900 to a 1920X1080 requires TWICE the number of pixel calculations! IF you are buying a monitor then I'd recommend a true High Definition (HD) 1920 X 1080, I "personally" prefer the 24" variety.
Bang for your Buck setup's:
(1) Single Monitor non-HD 1440X900 -> (1) HD 5770
(1) Single Monitor HD [1920X1080] -> CF HD 5770 (x2) or HD 5850
(2) Two Monitors HD [1920X1080] -> CF HD 5850+ (x2)
(3) Three Monitors HD [1920X1080] -> Triple CF HD 5850+ (x3)
Yep, people Triple CF/SLI GPUs to a single Monitor to achieve "wacko" FPS. If you can sustain 30-40 FPS then anything more is simply bragging rights - period! The Human eye can only process 25 FPS and see "jumps" in the rendering; the sudden jumps or lagging frequently is from loading lags from your HDD which is another reason Gamers like SSD (Solid State Drives) and/or CPU - bottlenecking.