Among other people on this forum, I will try to convince you to build your own computer. Custom-built computers will give you your requested flexibility, plus better value, performance and stability.
If you arent techniacally saavy, thats what we are here for. We can give you a list of parts to buy, and you can get your local computer shop to assemble the parts and install Windows on it for you.
But, to answer your actual question, you can remove the OEM label that you speak of, and its not very hard at all. No new chips are required.
Intel E6750...........PSN: ELBATROP
XFX nForce 650i Ultra
Patriot PC2-6400 8GB (4x2GB)
36GB WD Raptor
Samsung SyncMaster 931c
Samsung SyncMaster 750s
Windows 7 Home Premium 64