What is SA:MP?

San Andreas Multiplayer (SA:MP) is a modification for Grand Theft Auto: San Andreas which turns it into a multiplayer game. You can play over the internet (or LAN) with up to 500 other people. You need the original Grand Theft Auto: San Andreas PC game to play San Andreas Multiplayer.
In SA:MP you are able to do almost anything that you can do in single player GTA. SA:MP also features the ability to create your own scripted game modes. There are many game modes that feature a series of different and fun things to do. Many of which are not possible in single player.
System Requirements

SA:MP will run on any computer that will run San Andreas single player, but give better performance in most cases. However, if you are playing on larger servers(SA-MP Supports up to 500 slots), the game may run slower at times if you have a lower spec.
Minimum
1 GHz Processor
Pentium 3 / Athlon or equivalent
256MB RAM
Win 98/Me/2000/Xp/Vista
64MB Graphics Card (DirectX 9 compatible)
Radeon 8500 / GeForce 3 or equivalent
8X DVD-ROM Drive
3.6GB free HD space
DirectX 9 compatible soundcard
Recommended
Intel pentium 4 HT 2,8 Ghz Processor Intel pentium D 3.0 GHZ / AMD Athlon X2 3800+
1GB RAM
Win 98/Me/2000/Xp
PCIe 256MB Graphics Card (DirectX 9c compatible)
Radeon X800 / GeForce 6,7 or equivalent
16X DVD-ROM Drive
5.0GB free HD space
DirectX 9 compatible soundcard
SA:MP specific
In addition to the standard Single Player requirements, SA:MP needs the following:
5.6MB free Hard Drive space
An Internet Connection (512k+ Broadband is highly recommended for smooth online play)
A copy of GTA San Andreas for PC - V1.0 [NOT V1.01, V2.0, V3.0]