How do I tell what video interface I have

If you have a Pentium 3 or Athlon XP or better, you don't have to worry.

You'll have at least AGP 4x, and all AGP 8x cards can work in an AGP 4x slot.

So just get the best AGP card you can afford.

That's right, pretty much all AGP card nowdays support both AGP 4x and 8x. So you shouldn't worry about it

How about the video memory bus maybe 128bit or 256bit?How can one tell?
as far as interface
Wont the bios tell you ? Under advanced options video? the drop down window will say 4x-8x and what mb size? That was how i was able to tell on mine. It did not say 4x 8x it just showed voltages that i was able to cross reference to 8x. it was on pny webiste.
Speed Signalling Voltage
1x/2x 1.5V or 3.3V
4x 0.8V or 1.5V
8x 0.8V
Also How many Gaps the female connector has 1 is1x/2x 2 gaps is 4x

You can use CPU ID to get your motherboard info then get the manual and all specs from the MB manufacturer.

Memory bus is on the card. it has nothing to do with the AGP bus.

How do I get CPU ID? My display is kaputski without a video card.

CPU-Z will know that... just google for it.

What do you mean?

You want to know what CPU you have?

Well, if your video card's dead, and there's no backup display adapter, such as some other AGP or PCI video card, on-board graphics... You're really in quite a fix, then. As long as the slot isn't 3.3v, it should work fine with a modern AGP x4/x8 card.