Is this a pci express 16x slot?

What motherboard or kind of PC is it in?

Looks like it... Try it. Buy a card, see if it fits. If it doesn't then it's not aa x16 slot. Most consumer PCs have 1.

I googled photos and i'm 90% sure that it is a PCI E x 16 slot.

According to HP/Compaqs own site it is in fact a x16 PCI-E lane http://h20000.www2.hp.com/bizsuppo [...] =c00572527 so in theory you should be able to put any PCI-e or PCI-e 2.0 graphics card into the slot.

reason for the in theory is a bad experience with my neighbours Compaq that didn't accept components that wasn't approved from Compaq, however that was more than five years ago, so they might have changed the policy, either way you will most likely be able to return your graphics card to the vendor if it doesn't work so no risk in trying

Your good, just remember normally the system comes with a 250-350 watt psu. so check the specs before getting a new video card. 7600gt(low power with decent performance for most games, but no DX 10) or maybe even a 9600GT(good power/performance ratio) should be a good match for the system

Expansion slots (available)
Slot type Quantity
PCI Three (Two available)
PCI Express x16 One (One available)
DIMM Slots Four (Three available)