Hey I have the same problem with my Raddy. I have to set the AGP ap. to 32 mb or I get the O.pbo error.
This has nothing whatsoever to do with extra addons in my case as I keep my addon folder in pristine condition only extras in mine is the editor addon and a chopper or two.
On my GF4 I had no problem setting the AGP ap. to 128 but I did notice that If I had too many addons in my folder I would get the O.pbo. error.
It would also happen if I tried to set my own swap file, so I just set it on system managed and removed some addons and no problem anymore.
So what I learned with the GF4 was to do this, keep your addon folder clean and I would have a folder on my desktop with all the extra addons . When I would do a mission that requiered an extra addon I would just slap it in and off I went.
I should think you could do the same I mean really it is not as if you are going to use 768mb of addons at once....
As far as I know there is no work around for the Radeon AGP problem, but it is not really a problem m8, just clean out that addon folder and you are good to go
My main rig:
P4 @ 3.1
Abit IT7 Max2
512mb Corsair XMS pc3200 c2
Radeon 9700pro
2xMaxtor ATA133's  (Raid 0)
22" Iiyama Vision Master pro 512
Hope it helps
Gamer