No display after installing card

Video Card problems please help.. Ok so i am no noob to comps i have built at least 10 gaming computers and right now i am at awww So its a HP 9800 or 7800 i don't remember off the top of my head but my brother wanted me to fix it up so i put windows 7 with 4 gigs of ram and i had a Gforce 8600 Gt-OC and also a ATI X550 laying around so this problem is with both So, i put one of the cards in and you can hear the cards fans going so its getting power but all i get is a black screen unless plugged in stock port So i went to the Bios and changed it to PCI-E and looked through all the other settings in bios and found everything was OK. Then i went and got the new drivers and installed them After that i went to the start and in search typed. devmgmt.msc which this command will open a device manager which there i noticed that the card was not being recognized so i figured i would try to disable the on-board and see if it would kick er in but nothing. Now i am wondering if i may need a bigger power supply but on the ATI i know for a fact you don't its not the greatest card but better than on board. So, I am starting to think that just maybe the slot is messed up or something. Last but not least it did this to me when the computer was on XP as well so, can someone please help me very soon i am trying to do this for my little brother for Xmas
 
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