Hi!
Do you think that an Atmega328 can handle a LED matrix of 256 Leds via two 16-channel multiplexer?
(It would only have to display and listen to simple commands via serial or I2C to light the leds.)
I can't really test it, so I would be glad if someone could say if he thinks it's reliable? 
Edit:And would some processing power stay for a button matrix for example?
I've read 2-3ms/row would be ok?
From what we were told in one of the threads, the RGB shield, driving 192 LEDs, takes about 50% of the processor power. So I assume you can add 64 more LEDs, and it will still work.
Yes, but it uses some TLC5940N LED driver ICs...
It uses 74HC595 shift registers, one per mono-color LED matrix, plus one for (the common) rows, for a total of 4 (for the 8x8 RGB matrix).
Ok, there are actually more than one RGB LED shield I think, that's why I said that.
So it seems quite possible.
What's the shield you're talking about?
I was talking about HTINK RGB Shield
I was referring to this one, 8x8 RGB LED matrix:
http://www.seeedstudio.com/wiki/index.php?title=8x8_RGB_LED_Matrix_Driver
an handle a LED matrix of 256 Leds via two 16-channel multiplexer?
I would thought you would be pushing it. I used a straight 8 by 8 matrix in this project:- http://www.thebox.myzen.co.uk/Hardware/Econo_Monome.html and found the refresh rate was quite fast to prevent the display looking flickery. Basically you need the LED to be on 35 to 40 times a second, let's say 40.
Then you are going to have 16 rows so you need the refresh rate of 16 * 40 = 640 times a second. That's 1.5mS between refreshes.
Now remember that the current in effect gets divided by 15 because you have an on / off ratio of 15 to 1. So to compensate you would need to have the normal 20mA boosted to 20 * 15 = 300mA per LED. While this could be within the spec of your LED I doubt it. It also means that if ever the multiplexing stopped you would burn out the LED. Even at the equivalent of 10mA per LED you need 150 mA pulse current in the LED.
So I would say it's not quite on. 
Thanks Mike for your answer.
Hm yes, that seems a bit too much...
For the brightness of the leds I was thinking at about 9000mcd, but as you say it's only half the problem!
I have still to think about it...