To make sure that the card is operating at 2x go into BIOS and look for something that says "AGP mode" or something to that effect. You'll then have a choice as to what mode you want the card to operate in. The wording is different for different BIOS' but the result is the same.
Older motherboards will generally have a 3.3 volt AGP slot while newer boards will have 1.5 volt AGP slots.
Its true that the card will adjust itself to what speed the board is running at, BUT you have to go into BIOS to tell the board what speed to run at.
Also you might need to adjust the "Graphics Aperture." If you have 512 meg of RAM you can pretty much safely set the Aperture to 256. However, If you have less than 512, you might want to set it at 128 or even 64. 25% of your RAM dedicated to the Graphics Aperture is the rulle of thumb. If you set the Aperture too high you'll notice a serious performance loss.
Besides
www.tomshardware.com definitely check out
www.guru3d.com and get the Geforce Tweak Utility if its a Nvidia based card. I use it and its and awesome little tool.