Dell.com lets you configure your own computer (nice) but doesn't tell you the specific make/model of the components so you can't look at benchmarks/reviews, etc. for them. (Maybe that economical SSD drive is complete cr*p )
Often they (larger OEMs) won't tell you ahead of time because they want to be able to swap makes/model of individual parts without changing the over-all system. This allows them to buy whatever is cheapest and most available at the time, but still put out the same "model".
So a pretend Dell model like "Dell Optiplex with a 500GB drive" might have a 500GB drive by Seagate today, and tomorrow it might be a Western digital, but it's still a "Dell Optiplex with a 500GB drive".
If you want specific parts, I'd suggest buying the parts and assemble your own system (or go to a whitebox-building computer store and get them to assemble you one).