|If it will do what you want, Elements 9 is certainly a good path. So, IMO, is staying with CS2. I am still using CS3 at work, and for 99.5% of my needs it works just fine. Now, if I needed to convert RAWs from newer cameras, well, that's another story.
If you elect to move outside the Ps family, for instance to PSP, you will of curse have another steep user-interface curve to climb.
One thing to consider is that you are at a crossroads. You can still upgrade from CS2 to CS5 @ $199 USD, but that option will almost certainly vanish with the upcoming release of Ps CS6. I believe that CS6 is actually out for beta, or soon to be out for beta. The cost of upgrade to CS5 is not much higher than some of the alternatives.
Another possibility that you may not have considered is to watch for a deal on Lightroom. It will give you very powerful RAW processing, as well as a potent organizational tool. Its list price is probably more than you want to spend though, and you do need good, quick hardware makes the user experience pleasant (though I do use it on a 4-year-old laptop without much pain). You could then stick with CS2 for more complex edits. The Lr interface is another learning curve, of course, so consider that as well.
One final option (careful with this one!) is to watch fleabay or other online resources for a deal on a copy of CS3 or newer. CS3 will handle Camera Raw v 4.6, which supports the 5D; I just opened a 5D RAW file on my work laptop to verify.