I think the sickest aftermarket navi is the eclipse AVN6600
http://www.eclipse-web.com/6600index.html
The actual navi interface is extremly similar to the lexus navi's. I also like how it has the additional options like the back-up camera, HD-Radio, 5.1 decoder w/ center channel speaker, or the iPod connection.... It features the dual DVD players. Ohh and you cannot forget about the 5v preout ready for some serious speaker action.... i may look into selling my stock unit to hook this badboy up....