You'll have to recompile the nVidia drivers (well, the kernel-mode half, not the GLX half).
But you shouldn't have to recompile X. The kernel is still binary compatible with everything (that's why it's not kernel version 3
), so your current X binaries should work.
If you do compile 2.4.19, enable MTRR support in the kernel config. That should help framerates a bit, since the driver can enable write combining with that. It's under "Processor type and features", and it's named "Memory Type Range Register (MTRR) support" (assuming you are using make menuconfig or xconfig to configure the kernel).
I assume /proc/mounts lists /dev/shm as mounted, right? Is it listed before or after /dev? (should be after.)