5.1 or 7.1?

[Tom Bars]

my set up so far is a
47' LG
40gb PS3
and a lame sony soundbar

i want to upgrade to 5.1 0r 7.1...
heres my question, most of the blu-rays that i own say
'Dolby TrueHD 5.1' or 'DTS-HD Master Audio 5.1'
..so if i have a 7.1 setup for example
but the audio on the bluray says it supports 5.1 (like one of those i listed above), will any sound come out of the other two speakers.
its a newbie question i know, but if someone could briefly awnser it or kind of explain a bit better that would be much appreciated