Almost all video cards come in AGP format or PCI. PCI video cards are being faded out, however you still are able to buy PCI versions.
The motherboard decides what type of video card you are able to buy. Check with your motherboard manual to see if it does have a AGP slot. It there is a slot available then you should buy a AGP type video card.
if there is no AGP slot on your motherboard then you will have to get a PCI video card.
The minimum Memory size of a video card today should be 4MB at the LEAST. This will allow you to use 32-bit colour at 1024x768 resolution. Most 4MB cards will suffice for work issues and will not greatly improve the speed of your applications
However if you play alot of 3D games then a 3D card is in order.
A 3D card will handle all the 3D calculations, thus freeing up the CPU for other operations. It will GREATLY improve the speed of your computer when playing games.
There are many choices of what brand of 3D card to get. You will have to read reviews to see what the best video card is currently.
3DFX VooDoo 3 2000
A PCI and AGP version of this are both available
TNT2 Ultra or a Matrox G400
Both cards are excellent is 3D value.