Problems running games?

Hi all,

I have a ATI Radeon HD 4850 by MSI I don't think they make it anymore, all I know is that it had a special heatsink. It was an open box product bought from Newegg, and I can't find it there anymore either. It works well and doesn't heat up too bad (I can't tell you idle/load temps). I'm running windows Vista 64 (OEM for sys. builders), 8GB of RAM, 1TB of memory (2x500 GB).
I have a problem running all kinds of games:
-Tropico 3
-Cities XL
-Fallout 3
-Section 8

and a couple others that are just freeware

My question is, Why won't these games work? Is it my video card? Is it a DirectX issue? Is it Windows Vista?