Vista laptop, 4GB (maxed out when I bought it)
XP desktop, 1GB (I think this is maxed out)
Old PC, 512MB (I kind of throw whatever OS I want on it... right now it has NT4, if memory serves; haven't really bothered with it since I got the laptop, however- Virtual PC runs almost any OS at 100% speed on the laptop, so I can test my programs & installers without resorting to going through arduous install/reinstall processes...
Toshiba Satellite 440CDX, 98SE 144MB (maxed out)
Thinkpad 755CDV Win31 and DOS 6, 8MB (maxes out at 40MB, but there is no way I'll find the module at a reasonable price these days!)
Vista Laptop runs the best of all of them, of course, being my only dual core machine likely being the main factor there. Runs everything, except a few games, faster then my desktop, which has the sole advantage of a separate graphics card (FX 5500)