Runs alright on my system (A64 3500+/2gb/X1900XT 512mb) except for these massive and sudden frame rate drops. The frame drops seem to occur in certain areas or when looking in a particular direction. The odd thing is that these slowdowns do not always occur due to graphical complexity, so for example you could be in a closed room, looking at the wall and the frame rate will drop to 10fps. Other times you'll be in lag hell, but take a few steps forward and every thing is silky smooth.
Apart from that the game is excellent fun, but a bit easy, especially when you master the 3rd person peaking feature; taking head shots without even looking through the scope isn't my idea of realism.