Well I don't entirely agree that you couldn't run XP SP3 on 256 of RAM, but that's neither here nor there. XP also suffered from registry build up. The system literally needed to be reinstalled every couple of years because the registry would just get larger and larger. Windows 7 seems to have "fixed" this issue.
When comparing the latest version of Ubuntu to the Latest Version of Windows... I personally go for Windows, but that's mainly because I think Ubuntu has become over bloated
I guess it depends on what you want to use the r-pi for. If you're thinking about using it as a webserver, eventually even linux will slow down if you don't set up your log files right, but I'd still use linux for a webserver over windows.
If you're thinking media server, both a PC and Linux would perform equally, but then the r-pi wins out on the initial cost issue and the power cost issue and the maintenance cost issue ($35 wouldn't even replace a borken component on a PC, but would get you a whole new pi) which means your de facto choice is linux as the r-pi does not do windows.
Dear forum: Play nice