Anders, I would really recommend a new system instead of upgrading a system that's at the end of its life. You may see it as saving money, but it is only delaying an inevitable upgrade, and then you have sunk money into an old machine and will still have to purchase a new machine eventually. Not to mention that having two machines is a huge speed up for any workflow, you can have one box rendering or encoding while you work away on the other.
That said: I think 120 gb should be enough for a cache for the kinds of projects you have described. Instead of buying a 250, I'd spend the money on a second drive and run them in raid 0. This nearly doubles your I/O speeds. The only drawback is if one drive fails you lose the info on both. However if you are using it mainly for cache and temp projects, and back up the info to your main hdd that shouldn't be an issue.
The speed difference between a core i7 930 and the 3770k is immense. We are talking about nearly a 100% boost in render speed from the cpu upgrade alone...