unfortunately it's alot more complicated than that.. here's what you could do, and what will happen-
-you (somehow) buy a powerful AGP graphics card, but you find out your powersupply doesnt have the amps/watts to support it.
-you buy a new motherboard and...
----your processor doesnt fit in the slot
--- -or your pc crashes because that motherboard doesnt like your memory
----or your power supply doesnt have the watts to run a good pci-E card
----and your power supply doesnt have a power connector for a pci-e card
----AND most likely windows won't work anymore as its only supposed to work with that one motherboard it was originally activated on
So you're screwed, you'd most likely need a new copy of windows, a new motherboard, and a new powersupply unit, new graphics card and then there's no guarantee what you bought would fit into your case(high end graphics cards are about a foot long), you'd be looking at like $400. Best you could hope for is to find a AGP graphics card that will somehow fit into your pc as is, but then its likely to run fallout3 like crap. There's always xbox360.
My co-worker sort of went through this with his friend. He got his friend into Wow and his friends pc died.. so his friend goes out and buys a new dell, but then the monitor goes bad. So friend goes out and buys a 20 inch widescreen monitor, but the pc is too weak to post Wow on a 20 inch screen since the resolution is too high, then guy then decides he will buy a new graphics card but.. oops 250 watt powersupply and no x16 pci-E slot. So now the friend has borrowed $800 from his inlaws and my co-worker is building him a pc that'll run any game and will be upgradable /rolls eyes
fallout3 is awesome btw