20-30GB? That seems like a waste of 17-27GB. Did you mean 2-3GB?
I've started to stray from the old philosophy of many partitions, I them more towards lgarners suggestion of combining /home and /var. On many of my systems I just use /boot and /, but in production I'd certainly add another part for /var and /home as suggested. You don't want a user or a log file running the system out of disk space.
Also, modern paging algorithms no longer require the 2-3 times physical memory for virtual. With that much memory I'd only use 1x. You should consider putting a small fast drive in the system for swap as you don't want to be paging to raid. And if you're going to do that, you could just put the system on it, too. Or at least symlink /var/log over there, you don't need to take the write penalty of putting your logs on RAID.