I assume you have a board with an AM3+ socket if you are considering the FX. But I would upgrade the card first for gaming. The 550ti is a rather weak mid range graphic card. The HD 7850 2GB is a great card for the money and will breathe new life into your Phenom II 945. Your frame rate will improve dramatically.
Your video work would gain a bit more by going to the 8-core FX, but if it is an either/or choice, go for the card.