With regards to Multi-Step checkout vs One-Step checkout: depends on type of your business and what you sell pretty much. One-step worked better for 2010 Olympic games shop -
http://www.getelastic.com/single-vs-two-page-checkout/
Taking credit cards out of your site vs on your site may be no difference for a small business, but large companies see the difference and prefer to accept credit cards within their web-site pages.