This topic has been much discussed with different opinions...
My experience is that it is entirely media-related, and TY for example seems to give better burns at "rated" speed or slightly above. Some other medias, notably MMC02RG20, react exactly the opposite and give better burns @4x (on the 109, NOT on some other burners).
I doubt that your PC couldn't handle the writing speed: I've just placed a 109 in a 5 years old PC and it works fine @8x. Just make sure DMA is enabled both for your burner and your HD, and that you use 80 pins IDE cables.
As to the effect of the built-in buffer underrun protection on actual burn quality, I don't know, but I would assume that it does its job as intended and thus doesn't affect the burn negatively. I'd like to be enlightened on the subject too. :iagree: