Upgrading Asus As7-vm mobo

I want to upgrade my Asus AS7-VM motherboard. It currently has an
Athlon 1GHz processor. According to Asus it will take up to a 1.4GHz
Athlon or 900 MHz Duron, I don't see why there is a difference in CPU
speed.

I checked the local computer store and it is only possible to get
Duron processors, 1.4, 1.6, 1.8 GHz. Will a 1.4 GHz Duron buy me much
over a 1 GHz Athlon given the smaller on-board cache?

The Duron is a much later design, so will presumable generate less
heat.

I also wonder if I can run one of the higher speed Durons in this
mobo? The Asus doc only mentions 1.4GHz but maybe that is because
faster processors were not available at the time.

The Athlon XP processor doesn't seem to be any good due to the faster
FSB speed (266 MHz as opposed to 133MHz).

tia
David