Mwave, for sure. I recently bought most of an AMD system from them, but the memory just wasn't in the box. No how, no way. Uhh-ohh...
Dropped them an email. Their reply was apologetic, sent me a new module, no questions asked. Getting it right the first time is good, but fixing screw-ups in a flash is the mark of a really good vendor.
Other online vendors who have done me right include Azzo, Onvia, Axion, BTI, Computer123, Mushkin and 2cooltek. Don't know how well they handle problems, never had any with these vendors.
It pays to shop around and to check out unfamiliar vendors. It doesn't pay to get killed on shipping by ordering from a multitude of different vendors to build one system.