I recently upgraded form a Sapphire 4890 to an XFX 5850 and the difference is very noticeable. For example, with the 4890 and playing StarCraft2, had all settings to "Ultimate" but had to keep the resolution to 1680x1050 or below (or else the game would freeze) but with the 5850 can run the same Ultimate settings but at 1920x1200 with a hiccup. Also with CoD:MW2, was getting great fps with the 4890 but had to cut back on the eye candy or else there was noticeable tearing or stutter when the action got busy but again, with the 5850 no such issues. The XFX 5850 overclocks well (from 725/1000 to 765/1125) and also runs much cooler than the 4xxx series.
You can pick up the same XFX 5850 for $209.00
plus cash back if you do the rebate.
Hope this helps, good luck!