Dual Monitor Video Card

I am planning on buying a new computer and to save money I might just go with dual cards instead of one. I’m planning on getting Dual 1GB GDDR5 NVIDIA® GeForce® GTX 460 – SLI Enabled but I was wondering exactly how it worked. So far after reading I know that each card basically renders half of the screen but can anyone go into more detail?
**BTW I’m buying a Alienware comp, *good or not good?*
I am really big on multitasking things like Gaming/Videostreaming/Micro. Office/iTunes at the same time. I also need this computer to last me a WHILE because I’m definitely not rich haha. Do you guys think this graphics card will be good enough? Any recommendations?
***ALSO, does the dual cards work for everything I do? For ex. they only work together for certain games and not everything i do on my system?