OK, I searched more thoroughly the thread list and I found some useful information regarding my problem. Sorry about duplicating.
So as far as I understand, the bottom line is this: E7505 is (tacitly) supported by NVAGP so I shouldn't worry about upgrading my kernel. Also, I'll need to reduce the memory to 2GB since there's possibly a bug in the driver, although I haven't seen any explicit official aknowledgement. And last,
as long as the kernel's AGPGART is configured as a module, and not compiled into the kernel, nvidia's driver should work fine with the "NvAGP" option set to "3" (default).
Am I missing something?
Also, can somebody confirm that the memory size issue is related to a bug in the driver?