There are a few things you could do. It's not impossible.
If you overclock your CPU & GPU (Do not overclock too much it might over heat)
Then it should work a little better.
Close programs in the background that are running EG: Torrent Programs, Music programs & Skype.
You can use a USB modem for virtual ram, I did try but didn't notice any changes in speed. But maybe if you do it all properly and evenly you will get it a bit faster.
You can also get mods for SA-MP that reduce shadows ingame that cause it to lag.