Advertised contrast ratios mean nothing as each company tests their own monitors using different methods meaning that the contrast ratios cannot be compared between companies. Also any advertised contrast ratio is always exaggerated so it comes down to competent reviews and taking the look at the monitor yourself.
Really you do not need to upgrade your screen, but if you insist I would advice a Samsung P2250. It's cheap, (about 200$), FullHD 1920*1080, and has an excellent screen quality. 21.5 inches, very sharp picture.
but no, you dont need to upgrade unless you feel that your current screen is just not enough