Yes, this worked for me
I had a 32-bit 2.6.5 kernel from the original FC2 install disks running on an old AMD Thunderbird (K7) chip on an old motherboard for which I cannot remember the model number.
Then I swapped the motherboard/CPU for an Albatron KT800 Pro II and a 3GHz Athlon64, while reusing the drives, soundcard and all the rest. I rebooted on 2.6.5 and it (FC2's kudzu) auto-detected all the changes on reboot, and driver-wise removed the old devices from the old motherboard and reinstalled the new devices from the new motherboard. I only had to reboot a second time to get the auto-detect to fully work for the gigabit ethernet, so there was minimum pain.
Then I downloaded the latest 18.104.22.168-521 source from kernel.org and configured the options for my system (that took a while!), and built the kernel from the source code using the K8 (Athlon64 chip using X86_64) options. Installing that kernel from the make process automatically made a new kernel boot option in grub called 22.214.171.124-521custom. So I boot from that now and have no real problems. I did have to manually edit the modprobe.conf and local rcinit files to get the sata drive to be reloaded on boot. It seems the sata-via driver gets lost after booting, but that doesn't crash the system. You have to reload the driver and remount the drive.
After that, I installed the latest nvidia 6111 drivers which installed and work with no apparent problems. I have an XFX 5200 (geforce FX based) video card.
Note that I boot on an IDE drive and have a second drive as SATA for my data/video storage (still working on getting mythtv running). If you are trying to boot on an SATA drive then I don't have any experience with that. My SATA drive, a new Maxtor 300G unit ($238 at newegg.com), is NOT in any RAID or LVM or anything other than being an individual drive. I have the motherboard BIOS set to use the SATA port as any old IDE (non-RAID) port.
So I do have the old 2.6.5 kernel in 32-bit mode to fall back in case there are any nasty problems running in 64-bit mode. None have shown up so far.