Okay, i figured this out (with a little help from my friends). I ran memtestG80 and got a kajillion errors so it was pretty certain a problem with my GPU. I had a pretty decent one laying around (GTX 670) from my recently exploded PC, so I swapped it out, and now Fraps is showing me a pegged "60" FPS on the ride in. I can now burn half of that on mods! And probably more if I can get my new GPU RMA's.
c0c0c0's post in Horrendous Performance, Right Out of the Gate was marked as the answer