I bought a new pc (entire pc) for about 600 euros, and it's not quite out of date yet, it runs bioshock on high pretty sweet ,thoug not dx10.
A good enough gfx card would be about 100 euros, maybe 150, my x1600GT seems to be doing pretty ok still.
It's not too expensive, if you make sure you have a motherboard you can build on, you can just buy parts for it when needed, i cankeep upgrading mine for quite a while still, eventhough it's not yet necessary.
an AM2 or core2duo mobo is lik 100 euros or so etc. so it's not that bad really.
It just sucks if your mobo is out of date, because replacing that, and the ram / cpu attached to i, costs a bit more.