Everyone runs out of pins. When talking to LEDs, you have the obvious way, the clever way, and Charlieplexing. Outboard hardware is a nice thing to free up some pins (and respect chip source/sink limits). This fellow wrote a library to talk to shift registers in a clever and fast manner:
http://www.elcojacobs.com/shiftpwm/
768 total LEDs is not a video wall (divide by 3 to get 256 RGB LEDS), but it is a LOT. More than the EMSL Peggy AND with non-multiplexed drive: likely brighter.
I have a lot of thoughts about LEDs and clever PWM hacks but I think this library (and its low-end-of-acceptable-to-me 75 Hz update rate) will suffice.
Basics...not what he is doing: http://www.arduino.cc/en/Tutorial/ShiftOut
The STP16C596 LED Sink Driver and the Allegro A6276EA are 16-bit specialized varieties, to save some board space (but not money...)
One of 1000 discussions of the cheaper 7400 series chips and their limits:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1264477287