Cave Waverider

Update: Since I was now able to download the BIOS from the main post, I reflashed both cards with it (it showed as the exact same version I had, though). It didn't help. Still the same problem. Does anybody have an idea what's causing it and how to fix it?Update 2: I've just upgraded to Windows 10 and the issue seems to be gone. The system boots up fine with Legacy mode disabled now, fast boot works as well.

This could have several causes. Try re-seating and re-connecting any hardware you have, including the mainboard. Make sure it doesn't touch anything that could cause it to short either. Perhaps try to boot it up with the minimal hardware connected (Mainboard/CPU/1 RAM DIMM, video card, monitor, keyboard). You could also try a different Power supply.

Hm. According to GPUz and AMD Catalyst Control Center I already have BIOS 015.022.000.001.000000 (113-C42904A1-119) BIOS on both my 7970Ms in my Alienware M18x-R2 and it shows as UEFI compatible. However, whenever I disable Legacy Boot with them enabled, Windows starts but while I hear the starting sound, the "Starting Windows" screen remains on and I can't see the Desktop. I can even do things with my mouse and keyboard and notice that the Windows desktop is running in the background of the "Starting Windows" screen normally. I can also do a Windows shutdown, etc.. If I boot with the integrated Ivy Bridge Intel graphics, I can get to the desktop with legacy boot disabled just fine. Since the BIOS that is linked in the top post seems to be the same version I have, will flashing it help any?

Just to be sure, you are using BIOS A12 not A11, right? For some reason, A11 shows up as the "latest" on the download page for my Service Tag, but A12 is actually the latest if you go on the generic M18x-R2 download page. According to A12 release notes, it seems to have Windows 8.1 support and some fixes regarding that OS, so it might help. But if you're already on it, then I don't know either.

Generally, I'm not sure if the upgrade would be worth it. It might be better to go with a GTX 970, if your budget allows it. However, I'd expect the 750 Ti to perform smoothly in such games as CS:Go and have no trouble with them at all. Perhaps there is some other culprit. Are drivers, software, etc. up to date?

Just to check, are the games set to a resolution supported by your screen? Did you try to hook up an external monitor and see if the issue is present there as well?