Workaround for memory bug
Using Fedora Core 6 I've managed to workaround the 512MB only bug by passing "agp=off" as a kernel argument. I've successfully booted multiple times with 1024MB and 1500MB of memory.
For those you don't know how to pass a kernel argument.. right as the virtual machine is starting up there's a countdown screen, press the return key before boot continues. Hit 'a' to append an argument and just add " agp=off" to the commandline. If that works, you can make the change permanent by editting /boot/grub/menu.lst as root.
Hope this helps (it certainly helped me).
-Dave