I'm guessing go to the game website and check the requirements there. Unless they are different when you download them from steam. I really don't know, I don't have steam.
One of the three games (which I am downloading now as I type) says it needs directx 8 graphics or higher, my laptop says it supports directx 11 graphics but does that mean I have it?
If you really want to know if you can run a game, go to www.systemrequirementslab.com and see for your self. It scans your computer and shows if you meet the minimum requirements and a recommended.
If your computer is strong you will have no Problems, Steam does nothing special , it just prevents Unoriginal(Pirate) games to be played. When you pay Steam for a game , you can automatically Download and Install it. I think. There`s been a long time since i bought my last game trough Steam. Just Depends of the Computer/Laptop.
Try checking the specifications first. If you have Windows 7/Vista,
Goto:-
Start -> Right-click "Computer" -> Properties; You should have your specifications listed there.
If you have XP,
Start -> Control Panel -> System
You will most probably be able to run most new games.
Now, the Directx is a programming interfaces for handling multimedia tasks and Directx8 was last employed in 2001. If you've bought your laptop fairly recently it should have Directx10. So you needn't worry about that.
According to this site; http://www.notebookcheck.net/ATI-Mobility-Radeon-HD-5470.23698.0.html , you should be able to play those games just fine on the Low settings.