Judging from these fact that you only have 256MB on your Dell, I suggest that you feed it with more, since 256 alone would involve using virtual memory too often, resulting in lag...
My mum has pc with 1GB ram / dual core processor but its broke at the moment. Would that be any good to play GTA with mods on?
As long as you have a good video card, it will probably run SA on a decent frame rate, with minimal settings
With those specifications, that would run GTA better than you would expect. Besides, what can you expect from Alienware?