I have a MacBook Pro (15-inch, 2.4/2.2 GHz, mid/late 2007) that has had a few graphics card problems. The logic board has needed replacement a couple times, meaning we stopped using graphics-intensive apps, and it has just gotten slower, and the hard drive is becoming full (obviously that will need to be dealt with).
Are Mavericks's performance improvements (App Nap, etc.) enough to make it not slower than 10.6.8? Stability is key, and I'm not sure the graphics card can handle it.
With goto fail and the other bugs that have yet to be discovered, I'm not sure when it would be a good idea to upgrade.