"have seen several companies selling 'OEM' versions that are downloadable"
Sounds very dodgy.
If you get an OM veresion make sure you have it on a proper CD SEALED and with Serial Number.
"is it acceptable to buy and use these OEM versions"
Yes MS license you to use the disk on one machine , but it dies with the machine.
See if you can "borrow" a media center CD from someone and use that to format the drive and load on your machine BUT use your own serial number on the sticker at th rear of the machine. This is legal to do.