My computer checked the configuration and found that it can play gta san very well and not lag but when downloading and playing gta san it was lagged though it was still very good last time, although it installs 8 fps mods still lag !
Uninstall any game modifications and restore the original version of the game if you lag with mods as you mentioned. Also, playing the game whilst downloading something in the background will almost always make you lag so another tip to you would probably be to also not play whilst you're downloading or vice versa (to not download whilst you're playing).
Since the fact that you're using a Pentium E5700 and the onboard motherboard graphics (G41 graphics driver), that's one reason why you're having low FPS. Gameboosters, like Polarbear suggested, wouldn't even work in the first place. (Gameboosters are a gimmick and usually never work.)
Looking at your motherboard, the Gigabyte GA-G41MT-S2P, it can support a Core2 Quad (these can go low as $10 for a Quad Q6600), which is a cheap upgrade. If you can also find a preowned graphics card, which I suggest getting an AMD Radeon HD 7570 (GDDR5 variant, really cheap, $20 USD here), you'd be able to play GTA SA fluidly. The 2GB RAM is sufficient enough, but does haggle down the system if you keep running things in the background. I do suggest getting 4GB at least, but 2GB is manageable.
I understand different markets have different prices. I'm here only providing suggestions and probably the cheapest upgrades that someone can probably save from lunch money. Any LGA775 CPUs, except for the Quad Extremes, are really cheap.
