Thanks for the replies.
In general, I'd say this much swap is not a good thing.
Even if I do dumb things like this, as well as playing with 20MB+ images regularly? I have 3x HDD - would 3x SWAP partitions of ~350MB spread over each one be more efficient?
Even 2 GB of swap will give you problems, realistically. If you ever get to the point where you are actively using that much ram and are paging out to 2GB or 4 GB of swap, your system will probably be useless anyway, due to heavy paging to disk.
The system was actually pretty responsive still - as far as being able to browse the web/check email, etc. goes - eventually when it started to slow down switching between virtual desktops I tried the Ctrl-Alt-Backspace sequence, but this appeared to do nothing, so used (at this point I had a blank screen) the SysReq function.
Far better to let the oom-killer take effect earlier in the process.
I'm picking you needed to use this because the system was so unresponsive... and that was probably because it was spending most of time pushing things in and out of swap.
However, the oom-killer doesn't kill the system, or the kernel. It'll kill processes that try to request memory when there is no more left - which is why it killed the process "passkey-agent" in your pasted log snippet. As I suspect all of your processes were running under your X session, you could have killed X instead (via ctrl-alt-backspace, for example). This may not have left your system in a usable state, due to other processes having been killed, but it would have been safe to reboot it normally from here.
As mentioned before, apart from pretty heavy swapping (the SWAP partition was on a 36GB Raptor HDD, so this may have helped the performance a bit) the system was pretty useable for an hour or so after this had started :-) Thanks for the replies - they have increased my understanding a little more! Cheers, Elroy.