The 270X is the GTX 660's rival so the performance should be very similar. I just looked over the specs and the 270X does have more shadier cores than the 660 has cuda cores but not a whole lot more and the 660 makes up for that with nvidias drivers being much more reliable. The 270X does have 256 bit compared to gtx 192 but i read that the gtx 660 192 does not limit it noticeably when compared to the 256 on the 270X.
I would be torn between the two cards so i cant really recommend one over the other. The 660 has amazing drivers and in my personal experience is rock solid with no issues at all, while the 270X seems to have slightly better specs. The refrence cooler on the gtx 660 is not that great i would recommend this version of the gtx 660 if you buy it.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121660
As for 1440x900 i have no experience with that so i cannot say. Hopefully someone else can answer that but i would guess that might drop the FPS lower than you want.
If your intrested i am selling my GTX 660 SC on ebay ill include the link. I have taken excellent care of it and its a solid card. Take a look its a good bit cheaper than buying it new
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=171187660603