In general GTA SA is an outdated game which does not receive any updates at all, luckily the modders have been taking care of the game's optimisation so far. I heard that people with high end PC's even get lag in GTA SA sometimes, so don't think it's all your PC's fault.
Try to lower your GTA SA' settings. Go to options > Advanced > Display options or however it's called. Lower the visual fx quality, MSAA antialiasing. That could also help. Also as far as I know SAMP addon fixes your game a bit so you don't lag at certain points. Such as the lag you get at blueberry by the liquor store.