Yes.
Most AGP4X Cards will work on AGP8X.
But Not All AGP2X Cards will work on AGP4X.
In my case.
I have a ASUS p4s333 mobo with AGP4x slot and i was able ot place a AGP8X Inno3d GeforceFX5200.
<b>MY PC SPECS</b>
Intel P4 2.0GHz | 256MB PC2100 DDR SDRAM | Nvidia GeForceFX 5200 64MB | Integrated Audio | Asus P4S333 SiS645 | Seagate 40GB 7.2KRPM HDD | WINXP PRO SP1