The new build is pretty much finished, and without any major trauma!
I used the ATI auto tune to overclock the 4850 from 500 to 690 (GPU) and 750 to 1153 (Mem) and have been playing around with in-game settings.
The old rig - Sempron 3000, 1Gb DDR 400, 6800 Ultra gave 48 max, 32 av 0 min on the BD track with 1024x768 res, water 0 and no AA or AF.
So far the new one with same res, water 1, effects 2, 4xAA a 8AF gives 70 av 185 max 17 min.
All I have to do now is find some time to fly.