no post after graphics card replacement

My old graphics card heatsink/fan screws melted(nvidia 8500gt pci-e)want sure if gpu was fried, so got a new card(sapphire ati 512 4730). After installing the card me machine wont post i just get a long beep repeated every 5 secs tried re-seating card no joy. To be honest not sure card is 100% seated although it doesnt look bad maybe 1 mil higher on one side. After a furtle on the web people seem to think it could be the psu but i`m sure my colors-it 480w is capable, using both 6 to 4 pin power adapters on the card. Rest of the system Q6600, 4gig ram, asus p5nd2 se m/b, 500g hdd. If anyone has any ideas before i pull the m/b out to see if can seat the card in it better i would be grateful, many thanks.