I'm thinking similar things.Little odd because my brother gets very respectable fps on L4D2 with an ageing x1950 pro and an E6600, and your card should be at least ~50% faster. Suggests either driver problems or the beginings of a dieing card.
L4D2 is mainly CPU orientated though (hl2 engine :E) but still an E6400 should be fine on it. 8800GTS's were never the best but yeah it shouldn't struggle with it so perhaps it is coming to the end of its days
