So I am to abandon Windows once and for all and make Linux my main daily driver for at least 3 months so I can get a feel for it.
I have run Linux Mint and ubuntu on this computer before (i5 2500k, Radeon 6950) with no problems. For a couple of weeks ago I got a new graphics card Nvidia GTX 750 ti. Today I began installning Debian and all went fine until I booted into the system. Everything was so blurry and I had so many artifacts it was impossible to navigate. Tried Ubuntu Gnome instead. Although it worked better, it crashed every 5 minutes and was quite laggy. Also tried Linux Mint, it also crashed every now and then and seemed slow.
You can do Ctrl+Alt+F3 to get to a TTY (Ctl+Alt+F1 to get back)
If you didnt install anything youll be using the nouveau drivers, they are generally good these days, however Debian stable wont have the latest improvements they have made in recent months.
Its very likely its just not fully supporting the graphics card yet.
If you go to the TTY console and install the propitiatory drivers, your problems should go away.
The nouveau drivers are a community effort to reverse engineer the nvidia drivers. They have only recently made significant improvements in the driver in regards to 3D and OpenGL support regardless of the age of cards. So older cards should see a benefit from it, obviously the drivers Debian ships still has bugs. :(
The 2500Ks have Intel HD 3000 integrated graphics. Are you running a Z68 motherboard, perhaps? Z77? If I recall, most Z68 and Z77 boards have some kind of digital video out: HDMI, DP, or DVI. H97 and below, I'm not sure.
It sounds like @Eden has put you on the right track, but if you keep running into problems, give it a try. Intel IG has helped me troubleshoot many a GPU issue. Good luck!
Just ran a clean install of Ubuntu on the weekend, and I also found it much easier to use the Igpu until I could get the proprietary drivers for my Nvidia card running...