Worst excercise in future proofing I ever attempted was buying an an X850 XT PE AGP card for my S754 system. I spent $450 on that card, and it was the fastest thing you could get on AGP at the time. I had heard it didn't support ShaderModel 3.0, but was told that it wasn't a big deal... then came nightmare after nightmare. First was couldn't play Oblivion with HDR, then couldn't play Bioshock at all...
Also to the OP, I remember playing Half-life through software accelerated on my pentium 233 MMX laptop, because my graphics card was too small in the VRAM.
At least now graphics cards are more standardized in their features... but with the performance increases from each generation I think it really is best to consistently buy midrange, if that will meet your performance needs. I'm still running a 5770, and probably won't upgrade until there is at least a next gen replacement for it.