HD3870 gives nearly equal performance to 8800GTS 320MB and 10% less than 8800GT, but is much cheaper and uses less power, plus 8800GT is permanently sold out.
x86 over AMD64 because the 0-2% performance gain in most applications is overwhelmingly offset by the compatibility issues.
XP over Vista. Actually, Linux-for-office-and-internet-tasks + XP-for-games over Vista.
OEM means you can't install it on any other PCs ever and it becomes invalid if you change two components of the PC over time.
Ubuntu. Linux for human beings.
If you are interested in trying Ubuntu or Linux in general, PM me and I will answer your questions and help you install it if you wish.