RAID for everything!
(I'm experimenting with a lot of tips out there on performance, as the task is teaching me a lot of stuff, and as per the request in the original question, I'll make a separate post for each).
If you've more than one drive, you can set up RAID. The pros and cons of different RAID levels is well documented all over the place, so I won't go into it. Personally I have two drives so I'm really picking between 0 and 1 (though mdadm can do a form of 5 on just two drives, but I haven't tried it). Since there are things that can go wrong with a computer - especially a laptop that has a greater risk of physical mishap - that no RAID level can save you from, and you therefore need a strategy for dealing with risks that doesn't depend on RAID to save your bacon, (it won't save you if you experiment with a performance tweak you read on the internet and it makes things unbootable, for example) I decided to go with RAID 0.
The easiest way to do this for the whole system is to install from the alternate ISO rather than the desktop installer that lets you boot straight off the CD/DVD/USB into Ubuntu.
Select "Manual" paritioning. Divide up your disks so that you've partitions you will use on each disk. E.g. if you've two disks and decided to set aside 100GB for /home then you would set aside 50GB on each if using RAID 0, 100GB on each if using RAID 1.
Select "Configure Software Raid". Select "Create RAID Volume" (or something close to that, I'm not going to boot into the installer to check the wording). Pick those partitions you want in your first RAID volume, the type of RAID, and create it. Repeat until you have all of your volumes set up. (You don't need to put your swap on RAID, just give the two or more swap partitions the same priority in fstab and they'll be used together without RAID).
The assign filesystems and what's mounted where as you would with any installation and away you go.
A lot of stuff out there says you can only have /boot on a RAID 1 or non-RAID partition. I've had it on RAID 0 without any problems, which may be a matter of Ubuntu moves on, linux moves on, or BIOS moves on (and if its the last then your BIOS may not be okay with /boot on RAID 0).
The installer will install grub on all your drives. This gives one the benefit that if something stops booting on your "first" drive stops working, you can go into the boot menu and boot off the second.
hdparm does indeed see things has having been doubled in speed for me, and there's certainly a perceptible increase in speed on a lot of things too. I found in experimenting that the first part of booting (after the grub menu, when you've blank purple) seems to be slower, the second part to be faster (rarely time to show the animation now) and application use is faster - the greatest increase in performance of any of the tips I've tried so far.