you are talking about apple and orange in here. why apple is red and orange is orange? they are both fruit.
Palm runs on Palm OS and PPC runs on another OS.
320x480 is all you need, it seems to be standard since it introduced by sony long time ago(about a year, I guess). now all the palms seems to come out in 320x480 resolution.
for palm programs, it doesn't require much processing power and their program is much smaller in size.
When the PPC first came out, palms were still at 160x160, so MS required all PPC manufacturers to use a 320x240 screen (as a selling point, so people would buy PPCs instead of palms). Unfortunately, they haven't kept up with the times, and the Pocket PC OS still officially only supports 320x240. Toshiba uses some hacks, er, proprietary software, to get around the 320x240 limit - unfortunately, it means some programs won't work correctly with the higher resolution.
It's not about processing power. While not being the main reason, I'm sure battery life is considered when choosing a native resolution for a PDA's LCD. That, coupled with the generally brigher Pocket PC backlighting, was probably a factor in choosing the lower res.