The resolution you game at is dependent on the monitor. If your monitor cannot display higher that 1024x768 (or 1280x1024), then that is the highest resolution you can game at. If this is the case, then the HD5750 would not be worth the extra $26.
However, if you intend to replace the monitor in the near future or the monitor can display higher than 1280x1024, then you should pay the extra $26 for the HD5750.
-Wolf sends