If it says AGP1 then it's an AGP slot and you'll need an AGP card.
The white clip thingy is manufacturer dependant. Some have a pin, some have a clip.
Of the three ebay links, the first is AGP, the other two aren't and thus won't fit/work
PCIX != PCI
Get one with 512mb of ram if you can afford it, it will make a difference in the future