To clarify what Bruno said, It's definitely AGP. The hook on the end is a dead give away before even looking at the "keyed" slots.

Also if you have a look in a machine of that vintage you'll see a bunch of slots all the same which will be PCI and then the slot closest to the CPU will be set back a bit further from the edge of the board. Conventional colours would be PCI=white, AGP=brown or something else too.

Yes that's 3.3V only card (1 key on the left). But remember this is a replacement card bought as a spare and is not necessarily indicative of the card in the machine now.

Got a motherboard model?

Re-reading, you haven't even tried it yet. Plug it in like Mark says and let the card talk to the monitor. Yes the driver will only show what it thinks the monitor is capable of so is probably basing that off your old model. It will read the capabilities. Does the card have DVI? Use it if you can. Worst case it won't run at its native resolution and will be slightly blurry (pet hate of mine is people that do that...)