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It should at least work with 512MB. Then disable (I've blacklisted it) the kernel module intel-agp and you're done.
OK - after getting into it, I found a post with the solution: Prevent 'intel-agp' from loading. It works for me!
Same problem but with Ubuntu 7.04. The highest setting is 610MB for the virtual machine. I really need more, because I'm developing with Java6...