I can't speak for XP, but running Ubuntu 8.04 on my HP (compaq) 6910p with 4gb RAM is significantly faster than the Vista Business Edition that it came with.
I for one would like to second the "why ship it with an OS?" question. If the big reason why they don't give a deeper discount for not having to install an OEM Windows is "due to the customization [and]... to take PCs off the production line"... I don't get it. Just put a post-it on the box saying, "Don't Ghost this one" and ship it blank.
I have a sneaking feeling these manufacturers get a kick-back from all the shareware junk they pre-install. Otherwise they could knock the Windows Vista OEM $120 off of the price, use the SAME hardware, and sell it to consumers for more than the $50 discount they offer today.