K, a little help. What in holy hell is this slot?

Ok,

So this may well be a newbish question but it's been a few years since I actually took a new computer apart to add anything. I open this box, a Slimline HP Pavilion this morning expecting to find an AGP slot and a PCI slot, maybe two - and instead I am greeted with this.


So the question, stupid as it may be, is simple; what in the holy hell is that slot? And yes before you troll, I have Googled it - at length... And come up with nothing.

And that answered...

The next question I will ask is, can I stick a video card in there, if so, what do people recommend on a severe budget?

Help me internets, you are my only hope.