A second GTX would give you a much more significant increase in gaming. For light video encoding, your current dual-core chip will suffice. If you go into serious video editing in the future then a quad would be a good investment, but for the time being you'll get a lot more out of a second GTX280.
For single GPU, your current rig is about the best you can get.
A quad core will help those games that are CPU limited (possibly GTA IV, WoW when a lot of action is on screen and a few other games).
For dual GPU you will definitely get a significant gain with your current rig but to get the best from SLI or Crossfire, i7 is the kiddy.
Even though you cant get the full performance from SLI, you will be best off with another GTX280 for most games, but GTA IV might force you to upgrade your CPU as well if you want to run the higher quality settings!
Video encoding will benefit greatly from more CPU cores, but its worth looking at the GPU encoding tools.
What videos/codecs will you be using for your encodes?