I wouldn't upgrade the RAM, it will be useless in 6 months when DDR3 becomes what we now see DDR1 as.
Considering you have PCI-E your GPU would be portable not be a bad upgrade...but I still wouldn't do it, as I don't know how much of an advantage you'd see. The 8800 is already a much more recent piece of tech than your CPU, so I image there might already be bottle necking.
Put it in perspective. Fry's this week has an X2 245 and Motherboard for $69.99...
You could buy something like that and upgrade, but then your GPU would be holding you back, so you'd upgrade it, in the process likely requiring a new PSU, and eventually you start itching for DDR3 ram, and you've basically bought a whole new system. Small upgrades add up.