The i3/5850 is the better long term choice. As Maziar pointed out, you have the ability to install an i5-750 or other quad core option later on.
However, at 1440x900 the ATI 5770 is all you'd need. Anything more than that is frankly a waste of money, unless you plan to buy a new monitor soon. The 5770 will do decent at 1920x1080 (1080p) with some games, so it also depends on what you're playing.
I could see doing an i5-750 + 5770 as the better choice. Ultimately, the CPU will perform a lot better, and at 1440x900 the 5770 is more than adequate. The $150 difference between a 5770 and 5850 will easily pay for the upgrade from the i3 to i5-750.