hello:
i recently did work for a client that had internet secure. Upon review her account, I discovered that if osCommerce offers authorize.net, you don't have to install an internet sercure module. IS uses the same process and variables as authorize.net. You just have to change the URL and the account info.
One other thing to think about is who is hosting your product page.
I would do a comparisson of set up fees, discount rates, and transactions fees between the various gateways. Then I would check the contribution section and the forum to see if a module has been suggested fo the gateway of your choice.
Hope that helps.
New 
