[SDL] DGA question

[Dave]

I have XFree86 4.0 and I keep hearing about DGA being included and not
fully implemented.  Can someone give me a quick rundown of what exactly
DGA is and why I do want to use it or not?  I also want to know how it can
help my SDL apps and games performance and how I can take advantage of it?

I guess that's not a small question.

I will take hyperlinks for answers if people know good docs to help a DGA
newbie.

Dave