Thanks for the tips folks. Will drop my swap image back to the recommended level of 512MB.
I take your point on more RAM, not more linodes, but the NodeBalancer + 2 Linodes idea was in preparation for scale.
At some point I will need this kind of setup so I was thinking ahead. Possibly too early to be investing in it, but there you go.
For now, I have tuned MySQL & Apache based on the tips here:
http://library.linode.com/troubleshooti ... networking
Next up: fine tune our App to plug some memory leaks.