One has to be realistic with the hardware it has. As an example I run the tech demo very fast, except for the volumetric smoke, it gets too slow to be playable. I have a cheap HD 2600 XT with 512 Mb GDDR3 and 120 stream processors, but for the price, it's very nice. I don't expect to have all the effects at maximum, and it won't be necessary really, because a competent art construction (concerning models, textures and all) can still look great, using the effects wisely so that the engine won't run slowly.
My PC is mid - end:
Core Duo e2140 running at 2.0GHZx2
Xpert Vision Ati HD2600XT 512 MB GDDR3 Super
2 GB Kingston DDRII 667 MHZ
Tough the components are not great, they're very well balanced, meaning as an example that I run MOH Airborne with full specs with no problems. As for Crysis(only tried demo) I can run all things in medium(except for textures, that can be maxed, because of the 512 MB) at 1280x1024 with no problems.
If you want high performance you'll have to spend more money of course.
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