Testing to see if a used graphics card works

I bought a used computer on ebay it came with a radeon 4850 graphics card and said ready out of the box.  Plugged it up and of course black screeen.  Pugged it into onboard graphics and it works.  By the way i had to use a dvi to vga adapter.  My question is if I disable the onboard through device manager and try the card again and the card doesn't work(it does feel hot) will the onboard work again when I take graphics card out and plug it back into onboard even though I disabled it to try card.

If you disable onboard graphics you can get back to that if the Radeon does not work,  but you say it get's hot,  when things get hot right on boot it leads me to a bad circuit on the video board,  I have a HD Radeon 4650 1Gb and I am sure it does get hot from my abuse to it when gaming,  make sure the card is snug and tight also try a different PCI slot,  I don't know what your bios or computer model is but I disabled onboard graphics through my BIOS,  if you can just use the Radeon VGA to VGA port from your card to your monitor try that,  I have the adapter for conversion but dont use it,   I use two media cables DVI to DVI two of them,  one port for each monitor and I use two monitors and have no problems,  if it works that way we can go from there.