Depends what you want. I suppose the most useful thing is the ability to boost wireless power (although you do risk burning out the router if you go too high).
You can download dd-wrt for free from
http://www.dd-wrt.com/dd-wrtv2/index.php ,
They have multiple versions on there. You are probably better off going for v2.3SP2, and just pick the relevant model of router.
There is a Wiki manual at
http://www.dd-wrt.com/wiki/index.php/Main_Page
BTW, be warned, if something goes wrong during the installation, you risk destroying the router.