I would definitely not go above 1600x900 with that setup but really it's just a matter of experimenting such as:
- choose the pre-configured LOW, MEDIUM, HIGH settings (if they exist), then
- choose your resolution (1600x900 or lower likely)
- run FRAPS with VSYNC OFF while tweaking
- further tweak Shadows, Bloom, AA etc to meet a comfortable frame rate
With a lower-end card you may wish to experiment with DoubleVSYNC:
1) Tweak your game until you get about 35FPS with rare or no drops below 30FPS
2) close the game
3) Open RadeonPro and add the game EXE file
4) choose "Tweaks" -> Always ON (Double VSYNC)
If it works for the game it will SYNC to a refresh half that of normal, such as 30FPS to avoid tearing and maintain a solid 30FPS.
You just have to try it and see for yourself if it helps you.