Reduce the visual details level, most of these effects doesn't work anyways in samp (like motion blur or grass or plants).
try to put out Mip mapping.
and if all this doesn't help try another/lower resolution.
On XP SP2, try to assign the game only to one of ur cores.
Where's is the Motion blur/grass disabling option?
Options->Display Setup->Advanced->Visual FX Quality
try setting it to lowest, maybe it helps, then you can try test higher options for better Visual FX.
Do you know how to assign one core to the game only?
There's a way doing that with the Task Manager, but i'd suggest you get EasyToolz.
Get it here:
http://www.computerbild.de/download/EasyToolz-1586033.html?dl=1The site is in german, just click the big blue 'Download starten'-button.
The tool is partly in english, partly in german, once startet, there's a green-yellow bar in your system tray, indicating your processor usage.
Right click the symbol and search the drop list for 'CPU Affinity'.
Click on 'Hinzufügen' / 'Add' to add a new programm, search for your gta_sa.exe, click open, and tick the 'CPU' checkbox, then the core u want it to assign to, uncheck all other CPU's. Also check 'Automatische CPU Verteilung' / 'Automatic CPU allocation' and click OK.
Your game is now assigned to one core.
EasyToolz reminds that changes, so right click the tray symbol and click on 'Autostart' to let it start automatically with windows, so you don't need to do that changes everytime.
Works perfect for me, hope it helps you too.