AMD Athlon XP 3000+ @2.1GHZ
1024MB DDR Ram
ATI Radeon 9200 SE 256MB
Win XP SP1
2x Segate 160GB hard drives (with 4GB+ INF stuff on both

)
I run OGL with the following:
-glow -allslev -ship_choice_3d -ambient_factor 95 -spec -spec_exp 11 -spec_static 0.8 -fps -window
I get +30fps except in really heavy combat. If it gets too much I make the mission designer cut stuff so it works well on my system

This is with max settings in FS2 options and using the high res DDS maps.
I don't run using the most of the HTL models though, they kill my system in combat, hence why there's low poly alternatives for most models included now.
I've tried to keep it running on slower systems, but with a 9800 pro you should be fine unless the ram slows it down.