One more time "It's the cpu!!!"
Here's some benchmarks for 2 x HD4850 (PII X4 940 @ 3.5GHz, gpu stock clocks):
3DMark06 score 18,494
same cpu but with 1 x HD4850:
3DMark06 score 13,208
CPU 5,169 (about the same)
PCMark05 9,518 (always goes up a little with one card)
From your system:
I'd swap 4GB of your 8GB for more cpu.
I'd swap 1 x HD4850 for more cpu.
Still, some benchmarks from the current set up you have would put things in perspective.