If it helps someone.
I have older Dell GX280 and this card doesn't allow it to post. In fact, I tried it in other newer Dells and it doesn't allow them to post either. I did read somewhere that Dell's PCI-E slots are limited to graphics cards only even though I found official Dell documentation that talks about PCI-E slot capabilities being able to use it for network, graphics and other purposes.
Not sure what to make of it, I even thought that card was defective and seller was nice enough to send me another one, but new card does exactly same thing in Dell computers. Sort of disappointing.