Yes you can run separate slideshows, but as I said, control will be focused in one place (only one keyboard and mouse).
I don't really think that you have other options for what you want to do: the only single card that is able to display 5 monitors is HD 7870 eyefinity 6 edition, but it costs over $400.
The first option is buying two cheap flex edition cards - they can drive 3 monitors each, as long as they are connected via DVI (one HDMI port is used with HDMI - DVI adapter). So two of those cards could drive 6 monitors.
I don't really see any benefit for going with HD 7870 - I bet you will not use that system to play computer games. HD 6450 have more than enough power for slideshows.
EDIT: Branden was 3 seconds faster than me