Check the bios for an option to "enable 2x AGP support" or something like that. If it isn't there, check with manufactorers website to see if it actually supports 2x AGP.
If it doesn't, you'll have to upgrade the motherboard to get it. On the bright side though, there is actually no real performance difference between 1x and 2x (hell, there's barely any between 1x and 8x) so you really shouldn't even worry about it.