Forget about clock speeds, cores, and streams. Game performance cannot be compared directly to those statistics because the relative performance of them varies greatly with the architecture of different cards. Instead focus directly on reviews showing the performance of different cards. Note that the relative performance of different cards change for different games, game setting, and monitor resolutions.
For general information about upgrading video cards, I recomment How to Upgrade Video Card
Their page on "Criteria" provide a helpful guide in identifying system requirements and limitations (such as PSU power) and personal preferences:
Upgrading Video Card - Defining Personal Preferences and System Requirements
And the page on gaming and the following one on reviews provide information about using reviews - such as FPS - and links to helpful sites
Best Video Card Upgrades for Gaming
To get more help from this site, you should post information (make and model) about your major system components (Cpu, memory, PSU, and current video card) and game preferences - games, settings, and monitor resolution.