I have a Xtasy G4 ti 4200 video card, the fan is vibrating up and down inside its encloser and now chatters- the fan itself is held by 3 screws and i should replace it, but where can i go?? visiontek website is no help.. Any
suggestions would be very appreciated

Mine did exactly the same then burned out and i had to send it back :( I would recommend sending it back to where you got it from if it's still under warranty if not you need to get another fan like This one (http://www.thermaltake.com/products/chipset/g4.htm) HTH