Do you know what motherboard you have?
If not, do you know what CPU you have? That'll give us a rough guess.
Anyway, best way is to open your case. If you see a (usually) brown slot above the rest of the (usually beige) PCI slots, and it either has a card in it that says SIS, or has no card at all--you have AGP.
If there is no slot set aside from the beige PCI slots, you're out of luck.
If there is a slot, but it's black, it may be PCI Express.
Examples (cheers to Newegg, whom I'm leeching them from <img src="http://www.piratesahoy.com/forum/style_emoticons/<#EMO_DIR#>/happy.gif" style="vertical-align:middle" emoid="^_^" border="0" alt="happy.gif" />):
This motherboard has an AGP slot:
<img src="http://images10.newegg.com/productimage/13-131-532-02.JPG" border="0" class="linked-image" />
Notice how the little crosspiece in the slot is about halfway along but nearer the back (it will be towards the front in the case).
This motherboard has a PCI Express slot:
<img src="http://images10.newegg.com/productimage/13-121-259-02.JPG" border="0" class="linked-image" />
Notice how the little crosspiece is -very- close to what will be towards the back in your case.
If there's a card in the slot in your case, you can tell where that crosspiece is by where the break in the plugged-in-to-the-slot part of the graphics card is.
But actually, what would better serve is if you tell us your price range, and what you'll be using it for.
Only POTC? POTC and other games? etc.
{ If your price range can squeeze up to $65 or so, I'd very much recommend instead getting a 6200. ZZF has one for $68, <a href="http://www.zipzoomfly.com/jsp/ProductDetail.jsp?ProductCode=322281&affiliate=shopping" target="_blank">http://www.zipzoomfly.com/jsp/ProductDetai...iliate=shopping</a> }
There are of course ATI alternatives if you prefer them--you can get a 9550 for about $50 that should blow a gf4 out of the water.