Your verification that the E7505 should work with NVAGP encouraged us to try some addtional things. The problem was solved in the following way:
1. Recompiled the kernel without AGPGART support (it was previously compiled in monolithically - not as a module). I think this was the cause of the problem in the first place.
2. Set Option "NvAgp" "1" in XF86Config-4
We rebooted after this and it still didn't work. Then we decided to:
3. Reinstall the 4349 driver
I'm not certain why step 3 was required, but it was...
Anyhow, now it works and I am very happy - thanks.