Computer can't detect second GPU

[jimmyrustles]

Hey guys.
I recently got myself a GT 640 GPU. The main reason was so that I could plug in a second monitor (because my first GPU, a GT 440, only had one slot). The thing is that my computer won't detect my 640. A guess was that it's because they're not from the same series, but I always have a hard time wrapping my head around the cards having to be the same.
Another search led me to believe that I need to connect them together with an SLI bridge. The thing is that I can't find somewhere to connect it in either graphics cards.
I've tried to reset my CMOS, but that didn't help.

Specs:
<300 W Power supply
1 TB Hard drive
125 GB Hard drive
Nvidia GT 440
Nvidia GT 640
4 GB RAM

Thanks beforehand.

[patio]

Which MBoard is this ? ?
That also makes a difference...

[camerongray]

Why are you using both cards together?  Take the 440 out and connect both your monitors to the 640.