Tough one as you will have to be careful with the HD capacity. I would suggest to entirely empty the HD to do a fresh install. When you install Ubuntu Hoary first, you will need to resize partition for 4GB (128MB for /boot and about 1GB (twice the RAM for your system like 1GB for 512MB RAM, the rest is for "/"). Then you can install FC3 after and choose FC3 grub as default if you like.
Desktop CPU: AMD Phenom II(tm) X4 Processor 940 AM2+ - Memory: 8GB DDR2-RAM - GPU: Nvidia Geforce GTX 460 v2 - OS: Fedora 18 Spherical Cow x86-64 and Windows 7 Ultimate SP1 64-bit
Laptop Toshiba Satellite C650D - OS: Fedora 19 Schrödinger's Cat (preview release) x86-64 and Microsoft Windows 7 64-bit