To start I think NVidia cards are crap to begin with but that's just my opinion.
And the FX5200 card is from 2002. Unfortunately you are stuck with that card. What format are these downloaded videos? I don't think it has anything to do with your video card, perhaps just non Mac supported format/player.
Home: 2006.5 iMac 20"
Me: 2010 MacBook Pro 13", iPad Mini Wifi 32GB Black, iPhone 4 16GB Black
Wife: 2011 MacBook Air 11.6", iPhone 4 16GB White
Are you backed up??? If so, why? Psyllium Fibre is not free but will do wonders!