What are the best e-commerce modules/setups? Googling for "Drupal E-commerce" brings up a ton of old results, and I wanted to weed out all the garbage. I'm a regular on SO but I thought that I might get better insight/advice here.

Running Drupal 7.9 (multiple sites will be installed but running their own Drupal install), what e-commerce packages should I install (shopping cart and payment options)?
What are the pitfalls to look out for?
What theme do you suggest, that is geared towards E-commerce?
I have not yet built an e-commerce site on D7, so I cannot get really specific. 

That said, the obvious starting point for e-commerce on Drupal 7 is the Drupal Commerce package. Just start with the basic package, see if it fits your needs, and install add-ons if you need them. There even is a dedicated Drupal Commerce website with documentation, a forum and more.

There are a couple of themes that are specifically geared towards e-commerce, for instance Acquia Prosper and AT Commerce.