The R9 270X is probably a little bit overkill for 1440x900, but yes it does allow you generally to turn all settings up and get 60+ fps on the things that you play, so i don't think it is that much overkill that it is not worth it. The R9 270 is $30 cheaper and would do just as well for you.
Do those 2 cards would be my suggestion.
Basically a 2GB 7850 might be a possibility, but their prices are so close to the R9 270 currently that it is not really worth it. However if you can get one for around $130 maybe $140 it would be well worth it.