if you want a PC that won't freeze up on you then 1Gb should be plenty. anything less than 1Gb isn't very good for multitasking and so on. Also, if you want to speed up your PC get a program like CCleaner and clean all the junk out once in awhile. Also try cleaning out the startup items via MSCONFIG and that will help speed up things a bit.
Desktop: Dell Dimension E510, 2.5GB RAM, Intel P4 3.0GHZ, 250GB HDD, Windows Vista Premium. Might not be the best, but the old girl is still running good.