I've taken a liking to doing incremental upgrades, so my latest significant upgrade (a motherboard, CPU, and memory swap) cost me around $600-700. Not bad considering the dramatic increase in computing performance and when you consider that my old set cost as much as that did in early 2008.
Hopefully technologies like PCI-E and SATA remain stable enough that upgrades like that stay practical (so far so good!). AGP was a big, bloody mess. Nothing is worse than an expansion slot type that has a number of different slot types and several different voltages that it could run on. That would be like if there were a varient of PCI that used half the voltage and ran double the speed, but if you were to put an older PCI card in then your system wouldn't boot. Bad, bad, bad.