it will only work if your mobo says its AGP slot can do 4x/8x, there are some old mobo out there from the day dont just cant work with 8x. but most of the time 4x and 8x will work like you both said just reduced preformance.
If you are only paying $25 go for it. AGP is backward compliant but that 8x card will only have half the bandwidth. the older AGP cards will not work in newer systems because of the voltage differences and the new AGP pro slots are configured (3 openings at the AGP interface on the newer cards vs 2 on the older cards) differently and the older cards will not fit into 4x/8x AGP slots.