humble I'm with you on BIOS flashing, use extreme caution. Mobo drivers, OTOH, are something I always used to keep updated when I was running VIA chipset boards (I've had several of 'em) and was always pleased with the results. I'm on my first Intel chipset board in years now, no opinion there yet.
I've never really looked at frame rate since I got my latest box running AH. Its simply worked well enough I had no reason to. To add to this discussion, I've just been doing that.
The lowest I could push 'em looking through a Panzer tube on full zoom was 20. That's with a full set of trees and a couple of brick buildings and fuel trucks all in the view.
I'm running a current P4 3.0GHz (6XX series) on the Intel 925X chipset, with a gig of PC4300 DDR2 SDRAM and a GEforce 6800 (not the GT, plain vanilla) video card.
Looking at published benchmarks, I'd say you have a slightly (10-12%) better video card than I do. My memory is somewhat better than yours, in terms of speed, but I wouldn't think that would affect framerate much. And I would suppose your CPU compares favorably to mine.
So, IMO its back down to setup or something.
I'm running the bottom two graphics sliders full left, and the top one somewhat right of center (*just* left of the "M" in "More Performance").
Playing with it, I found that moving the bottom two sliders toward "More Performance" made relatively little difference in frame rate, until pushed to extremes. OTOH, moving that top slider even a TAD more left caused a significant drop.
I have no complaint with the way the game looks. I do see that "trees covered with vines" effect you mentioned, but it goes away before I get very close to 'em - I can drive an M3 at full speed toward 'em and see the gaps open up in PLENTY of time to decide where to weave through 'em. Other than that, I don't see any anomalous video effects. The game simply looks beautiful.
I hope this helps.
culero