One thing that matters is how well was your Windows OS maintained. Scanned for mall ware, spyware, viruses, etc and defragmented or what programs run in the background (winamp, yahoo messenger, antivirus programs) while you play BS. All of these things are eating resources.
I have an Intel Q9550 with GForce 8800GTS, 4Gb of Ram and WinVista. BS2 runs with +35fps in a mission with a sunny day, and ~15fps when it's cloudy.
I scan my pc regularly and maintain my Windows OS as clean as possible. Also I have to say that the install is almost 2 years old.
Keep your Windows clean and BS2 can run with ok FPS.