I had, before chosing avast, done similar tests. The EICAR test file packed and encrypted with several formats. The files were downloaded from http://www.attac.net
I had a total of 27 different file formats and the result for each AV tested (number of files detected) :
Avast : 22
PC-Cillin : 22
NOD32 : 17
Antivir PE : 12
AVG 6.0 Free : 8
Since this test i switched from AVG to Avast! 8)
Note : the scan was done with on-demand scanner (explorer extension).