It should go up to nearly all of your RAM, then sit there, barring running and closing memory-hogging applications. While it defaults to some automatic value (sqrt(lowmem_kbytes*16)?), Linux keeps a good few MBs free--really free--and does its best to keep all the rest of it in use. I was unable to even change the GTK and KDE themes without programs crashing. Nepomuk still hasn't fixed its 100% CPU use bug, either, and still likes to crash (I have the same issue on Arch--it's not Ubuntu causing the problem, but software known to have such problems should not be included yet). PCManFM likes to unceremoniously close when transferring files over the network, too. It's not something new: 9.04 was the last just-released non-LTS Ubuntu I found good enough right after release. They'll get it fixed up, but that can mean GBs/week of updates, and some in-use annoyances, for now. 12.04 LTS, OTOH, is rock solid. All it may have issues with is a slightly old kernel (a problem for virtualization on IB, FI), but anyone likely to encounter such problems aught to be capable of acquiring or building a kernel. Pretty much everything but the kernel itself gets good backports, and there are PPAs for newer kernels.