Maybe I can help fellow 'burb man! I posted the same question some time back when I was looking to expand my system and only received one response so I did a lot of hands-on homework myself. I assembled my base A/V/Nav system before the market flooded with so many choices. At the time, only one source had both things I wanted --> touch screen and DVD navigation. So, my base is the Kenwood KVT-910DVD head coupled with the KNA-DV2100 nav system.
With my wife joining me on more and more road trips it was time to expand as I wanted to have the nav system, but wanted to allow her to have video entertainment. However, Kenwood has nil for add-on screens, and I was looking to add to the visors first. I found Power Acoustik 5.8" (PT-569MSV) and PYLE 5.6" (PLVS-MN56) screens at very inexpensive prices and ended up ordering one of each to have a look at them in person. Well, glad I decided to try them out before installing because you get what you pay for! At best, I would rate either as "fair" - with very faded pictures and lacking crispness. I then found some XO Vision 5.6" (XO256) units at slightly higher, but tolerable, price and ordered them to try out. Found them to be more in line with what I was looking for so installed one in each front visor.
I then decided to expand a few months later into the rear seating area. First stop was XO Vision but none of their larger screens produced the picture quality I wanted, especially with glare... On the advice of members of a Suburban forum I also visit, I gave VizuaLogic's "Perfect Match" replacement headrests with integral screens a shot. After shelling out $1250 (OUCH!) for a 2 headrest set (they were leather-wrapped) I was expecting something fantastic... Well, they weren't! The assemblies themselves were of excellent craftsmanship and a perfect match to the stock interior of the 'burb. However, the 7" monitors were nothing to sing about. Back they went as fast as they came... I then checked out the Alpine TME-M760 at a local dealer and was in awe of the picture, WIDE viewing angle and performance with glar, but most of all I was in AWE of the $650 price (best I could find at the time and it was on-line - dealer had them at $800)!!! So then I looked toward the original mobile video makers - Rosen - their LCDs were in airplanes before anyone was even thinking automobile applications so why not... Well, picked up a less spendy pair of Rosen Necvox FD7569s - paid $750 for BOTH with 2-day delivery from the left coast. I am VERY, VERY pleased with the Rosen units! Passengers, including my wife (who absolutely hates riding in the back), fight over who will be sitting behind the Rosens!!! Best bang for the buck in my case and you might be able to get them in your budget. Just my experience if it helps! Good luck!