I need a good PC game demos.

I agree with above. Also keep an eye on Origin as they will give away older games for free. I still haven't played bejeweled yet, but I know it's there if I want it.
 
But I want a demo of a game that have next generation graphics.I went to steam but there are no demos that have next generation graphics.
Steam has a lot of Indie game demos so you have to search for long enough until you come across a demo of a good game.
 
You can buy full games cheap on ebay, G2a, Kinguin etc

For example Metro 2033 is a pretty demanding game that you can buy for £1 in the UK
 
If your looking to test out your system 3dmark has a free version and so does Catzilla (which if you register *free* you can test @ a higher res).