Maybe what you need are up to date graphic card driver. You shouldn't lag with a computer like that. Go to Nvidia's site and download the latest drivers.
My computer is not too different from yours, the major difference is that I have a P4 640 (3.2Ghz) and my video card is a GF 6200 256/128, LC runs like a charm, at least when I was playing it... I use the latest drivers.