I'm one of the founders of Chargify. Whether or not you choose to use us (our focus is the management of recurring billing, products, coupons, proration, etc.), here is an article I wrote to help startups and SMBs understand payment gateways and merchant accounts:
If you just need to accept credit card payments of any sort, this is the foundation you'll need. PayPal Website Payments Pro is their version of gateway + merchant account, but frankly, after seeing hundreds of merchants choose this or that solution, I recommend other gateway/merchant account combos.
I hope it's helpful info.