Like the above poster said, you should clarify some infos in order to get the best advices you can have!
But without knowing your system and the games you play, i'd suggest, in the range of 120-150$ cards, the Radeon HD 5770 1gb.
Nice card with lots of features, directX 11 support, enough power so you can play most games in high resolutions and settings (linked with the resolution you play).
You'll need a motherboard with a PCI-Express slot for any new cards you'll get.
As mentionned, post some specs and infos and you'll get a bucket of good advices!