Shouldn't be a big deal at all. Back up important data first then make sure you choose the correct partition for the installation. You know the rest. Fedora will install the boot loader and you'll need to manually add the existing Fedora as a menu option if you intend booting that one also. When the time comes, if you don't know how, we can help set up grub to boot the already installed 64Bit version along with the other 2.
The Bassinator © ®
Laptop: Toshiba Satellite / Intel Core 2 Duo 1.73 GHz / 2GB / 160GB / Intel Mobile 945GM/GMS/GME/943/940GML Integrated Graphics
Desktop: BioStar MCP6PB M2+ / AMD Phenom 9750 Quad Core / 4GB / 1TB SATA / 500GB SATA / EVGA GeForce 8400 GS 1GB