Hazael, I have seen this question over and over.From what I have read asa5505 does supports dual ISP but as a backup measure, your requirements of load balancing falls under policy base routing which asa/pix does not support. To load balance using pbr you will need to deal with complicated bgp dual homming configurations by utilizing router between ASA and ISPs, not to mention other requirements such as inbound traffic using different public ip blocks natted to a single host inside etc..
however, if traffic is simply outbound you could do pbr at router between asa and ISPs.
In any case , this link shows the basics for dual ISP configuration ASA/PIX.
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_configuration_example09186a00806e880b.shtml
Rgds
Jorge
Jorge Rodriguez