An HD 6850 is within your budget and would be perfect for your resolution.
i may have to decrease my budget a little if i go with a quad core i5 instead of an i3. Would the cheaper cards, i.e., 550ti, hd6790 and hd5770 be able to max out games (or run on pretty high settings) at 1600x900? And should i go for a cheaper card + core i5 2400, or a hd6850 + core i3 2100? i have heard that CPU helps at lower resolutions. Any suggestions???