click here... this should help you.
If your gaming, you should spend most money on the graphics card. Personally, I would say get an Nvidia 8800 series graphics card. 256mb if your gaming on a 19" monitor or less. 512mb if your 20" or higher. 256mb can be had for around £130 and the 512mb versions are around £170.
SLI is when you run 2 graphics cards. ATM, no games take advantage of this.
Hope this helps