Burning failing with that RAM amount has got to be something else than the cache (or rather, free RAM). I've burned unsuccessfully with a bad (read: old) drive with lots of (free) RAM, and after that successfully with a new, good drive with very little (free) RAM.
As long as you have free RAM available, it shouldn't be a problem. 2G is enough for home use if you ask me (servers run happily with 2G at many workplaces); maybe you should find out what exactly is the real reason behind failed burns, for example. It could
be the memory issue you're referring to, but I doubt it on behalf of my experience.
Sorry that I can't actually help on your last-line question on how to free the cache (except for a reboot, or killing the processes that reserve the memory), but I'd just think the problem from another perspective. None of my personal machines has at the moment 2G of RAM, not even 1G, and all of them burn cds and dvds happily with even less free RAM than you have.
I hope you're not using amule for illegal purposes, because in that case I'm just happy if your machine gets stuck