I'm getting ready to FINALLY buy a new vid card, a 6800GT. I was wondering how I tell whether my comp has an AGP or PCI slot. I currently have a 5200 FX 128M :-/
A PCI slot is always white and close to the edge of the motherboard. There usually will be 3-8 of them aligned next to each other (unless you have a mini-ITX board...like a shuttle computer or a Mega651). Soundcards, TV tuner cards, and others will use this slot.
There will only ever be one AGP slot on a motherboard. It will be set farther back and is brown/dark tan in color. Only Video cards can use this slot.

NOW WAIT! Before you rush out an buy a new card, there is a catch.
See, PCI and PCI-Express are COMPLETELY different! If you are looking to buy a PCI-Express video card, you need to have a motherboard that supports PCI-Express. You will want to kick your own rear end if you try to plug an Express card into a regular PCI slot...it won't fit.

You can see three regular PCI slots here. The black slot is for PCI-Express (16x) and the tiny slots are for PCI-Express extenders (1x)