It would be benificial to your post had you have added your current system specs.! Without adding your SPECIFIC SYSTEM SPEC'S, you will get advice that MAY OR MAY NOT reflect capabilities/compatabilities of your current components! BEWARE OF BAD ADVICE, speculation is a BAD IDEA!
The 7850 or the 650Ti would be the best options for you with certain games going over the 1 gigabyte in ram mark I would look towards the 2 gigabyte model to hold you over a bit more. I've gone ahead and looked up both cards in there 2 gigabyte setups and sorted them by price so you can look at them.
You didn't supply enough information for me to actually give a highly educated reply! Sorry but if you want more sometimes it's best to give more! Full system specs & the purpose & even the game or games you intend to play determine various results!