Magento

I am just signing up for a merchant account. I have one company (making t-shirts), but I am using multiple web sites because I have multiple lines of shirts that I market differently. The company I’m using (PayMerica) told me that I should set up a single virtual terminal where I would hand-process the submissions, because if they set up a separate gateway for each site, then I’d be paying a separate fee for each site.

They offer a cart called “ZenCart”, which honestly looks like a piece of crap to me. I just discovered Magento, and it looks amazing to me.

My two questions are--
Does Magento’s multipe-site capability make it possible for me to run my store on different web sites but then have the transactions all go through one gateway, so that I could just set it up that way and avoid the hassle with the virtual terminal? If so, can someone point me to documentation which would help me figure out how to do that?

Second, I have been using Joomla CMS on my sites lately because I have pretty much zero time for developing them. I like the fact that there are so many extensions already built for Joomla, but I’m not dependent on them. Would there be any point in trying to integrate Magento into Joomla for an e-commerce site, or should I just build the entire sites using Magento? If yes, again my question is, can someone help me out finding info on how to do that?

Thank you.