For most apps & games nowadays, 512mb is the minimum for good performance, in my opinion. It also depends on what software you're running.
Battlefield 1942 is a good example, although a bit higher in mem usage than most games. The maps load up faster on my 950 Athlon with 1G pc133 SDRAM compared to my P4 1.6@2.13GHz with 512 DDR running at 354Mhz. But once in the game, it runs a lot better on the P4 (which will see another 512mb when the $$ comes
)
I personally like to have more RAM than I need, and like minimizing swap file size & useage.