I get this problem, for me, it all orginated when my old Motherboard was replaced by the mechanics, they replaced my 800MHZ FSB Motherboard with a 200MHZ FSB Motherboard, I bought some new RAM and got Graphics Card and it fixed the lag in most of my games, however, San Andreas is still drops FPS, and I can't run Firefox, Photoshop, Media Player, and and Messenger all at once without some lag.
Your system settings are similar to mine, so you will proberbly need either a new Motherboard or Processor or something.
I have Intel Pentium D, its a very fast dual core processor at 3:00GHZ, however, it isn't a durable one.