With a system like yours running at default speed with default settings, I'd say your 3D score is right where it's supposed to be. Now to get higher.
How much you can overclock depends on your CPU temps right now and if you have good case ventilation. Usually, you can O/C with stock cooling as long as you don't raise vcore much.
Exactly which CPU do you have? You said E800, but that must be a typo. Since you said it runs stock at 3 gig, I'll assume it's an E8400. Try 350 FSB and see what happens. That will give you 3.15 gig which should be no problem to get you started. I suspect you can go higher, but try that first and check temps and test for stability.
The ATI Catalyst Control Center (CCC for short) is where you can change video settings for ATI cards. Go into CCC and move the 3D "quality" slider all the way to the right (see pic).
Here is a nice tweak guide for the CCC.
http://www.tweakguides.com/ATICAT_1.html