Getting a GeForce GTS 250 to work with Windows 8

Hello,
Santa got me a new HP Envy for Xmas with Windows 8, and for video it came with a Geforce GT 620/1GB DDR3, that just doesn't look like a whole hell of a lot. It's a slim-line or half-size or what ever the form factor is called, and it just doesn't seem as impressive as my older GTS 250 that was running in my Vista x32 system. It's a nice big 2-slot beefy thing with a massive heatsink and fan. I've never used it in a 64 bit DX11 environment, so I wanted to trade it out with the GT 620 and see if it performed better. Problem is, when I change cards, I get a black screen and a beep/LED code that translates into 'pre video graphics failure' according to the HP site, and they won't talk to me now because I violated the warranty when I tried switching cards. The code is BS anyway, I put the cards back in the Vista 32 comp and it runs fine. I'm not giving up on this though, so where do I go from here? I've tried rolling my Nvidia drivers back to the 306 and 304 legacies which did nothing at all. I'm getting the feeling that I'm gettting blocked by the UEFI, of which I only have a basic 'how it works' understanding, I don't know how to manipulate it, but I suspect if I could flash the GeForce BIOS to a newer version it might satisfy the UEFI and allow it to boot. So please, someone tell me what to do and how to do it