Yes, it will actually just use memory you dedicate, sometimes even less, sometimes a bit more, depends on your settings, how much video memory, mounted stuff and so on. VirtualBox itself takes 30 + 5 MB on Windows system. I guess it's something similar on other systems. Memory you can see from the second screenshot above -- it was 1,4 GB for 3 virtual systems ( 256, 512 and 666 MB ). It could be less, both Linuxes run OK with less than 512 as well. XP is using only 70 MB or so, because it's optimized edition, but normal one can run OK on older systems with even less than 256 MB RAM.
I still need to explore diff settings, especially PAE/NX and Nested Paging, 3D acceleration and more than 1 CPU. But, so far it runs very well, seems very stable and i personally like it more than VMWare, but i didn't benchmark it or anything, just my preference.