What version of 3dmark is this?
For a reference, I have seen ~14K+ in 3dmark06 with a mildly overclocked E7600 @ 3.0ghz/4850 (overclocked also) - so a comparable system.
Have you played with the GPU speeds?
That score isn't that poor. One thing to keep in mind when running 3Dmark benchmarks, they get mostly higher end, enthusiest type computers that run their benchmarks. The average computer user would never download their software.
Essentially, their "common system" is probably much better than average computer in the real world. Look at their "common system" specs. It lists a i7 920 with a GTX 295.
About par for a 4850 with a CPU running at 2.8ghz. If your CPU is running at 3.4ghz, you should be getting better. Do you have cool and quiet disabled? It is not supposed to make a difference, but it will.
Look at my specs. My score is in the lower 14's.
I can hit low 15's if I overclock my GPU. (8800GTS)
Vista will give you a lower score than XP though, and I am running XP. But still it looks kinda low to me. I would think low 14's would be doable at 3.4ghz, even on Vista.
I just saw something, your x2, that might have a little to do with it. 3Dmark 06 will favor quads a little, as the CPU part of the test is really tough.
So the difference is probably a lot based on that. But your Graphic performance may be right in the ball park.
But anyway, how does it do in games? That is the real test. Use the benchmark for your own use if you do upgrades, but don't put too much faith in comparing to other systems that have run the benchmark.
My 4850 easily runs at the max overclock that CCC(catalyst control center allows) , 700/1200mhz. I would try working your way up to that inside CCC for starters - that should put you over 15000 marks.
My 7300 (7600 was a typo) @ 3300mhz with my 4850 @ stock clocks made for 12156 marks. With the same CPU speed, but OC'ing the 4850 to 700/1200 put that up to ~13300.
With my CPU clocked near yours @ 3409 , and the 4850 @ 700/1200 - It was over 15k marks!
Worth a shot - CCC tests stability for you in about 30 seconds.
I tried O.C HD4850 using Auto-Tune in the CCC... CCC after testing at different frequencies found out best suitable... but that frequencies made crash while running Games/3DMark06.. I don't know why this is happening... I think my GPU was not made to O.C... Its a MSI HD4850..