I can take a stock Dell install and clean it up in about 20 minutes. I wonder if your question is about which 7 to buy or is it about "clean install"?
The license changed a bit over the years so I will not enter the OEM license debate. Just get the version of 7 OEM you want at newegg, etc. and go for it.
I currently have windows vista on dell xps 730x,Whcih windows 7 disc should i buy to install a clean copy of windows 7 on my computer? I am asking because i just happen to install windows 7 on my friends computer from windows 7 OEM disc, after installing that the authentication code would not work.