Upgraded to Sapphire 4670 1Gb, silent fan, and mediaportal is happy again.
That is $150 the mediaportal team won't be getting as a donation. :-)
P.S. Sapphire 4670 Ultimate does NOT FIT in a Antec Fusion 430. Heat sink on top of the card is about 1cm too high to get the case lid back on. It was suggested I should go the "mad-max" route and have the sink protuding from the top of the case... Nah.