agp_try_unsupported only works with the agpgart driver.
I'm not positive whether the nvagp driver contains support for the KTxxx series chipsets, though. I know some have had trouble with the KT400, and I think some others have had trouble with the KT333 (which is just a KT266 with a faster memory bus), but I could be wrong on that. I have a KT333, and it works just fine with agpgart (kernel 2.4.19 with the -preempt patchset).
Why did you disable agpgart? Does enabling it (and recompiling the kernel, obviously -- if you make it a module, you may have to do something like post-install nvidia /sbin/modprobe -k agpgart and pre-remove nvidia /sbin/modprobe -r agpgart in your modules.conf to get the module to load when it's needed, though) help?