What version of ubuntu are we talking about here. Granted pretty much all the versions are extremely low on resources.
As a reference I set up and have been running Ubuntu 6.06 on over 20 machines in a private IT room at school. These machines are quite old, specs are AMD K6-2 some are around the 300Mhz mark some are just about 500Mhz, it runs ok on these. All of those have either 128Mb or 256Mb RAM in them, but al the PC's are actually able to run it all fine. (a bit of lig here and there)
So in answer to your question, any type of processor (within reason) can run ubuntu, there arent really any special requirements.
And also, obviously, no it doesnt take up as much resources as XP, certainly not vista. You have to remeber that XP are bulky fleshed out OS's to cater to the most dim-witted individual's needs. Ubuntu, as with any Unix core is literally just that, its an OS, it does what you tell it to, and it doesnt help you out very much along the way. Sure ubuntu is probably one of the most helpful linux distros around, but still, it can never match up to the simple-ness of XP
What are you going to be using ubuntu for?
What's your budget for the system?
dragon2309