I would not go with an X3 and stick with X4. You may not feel the difference with the current games but in the next generation of games (you seem to keep the computer 3-4 years) and definitely with media encoding today, you will feel the difference.
Since you will be targeting 30-60 days purchase window, you can still stay in the $600 budget if you order the parts as you catch deals on the internet. Also, it will help with catching those deals if you have alternative brands for memory, motherboard, and graphics card in mind. For example you can keep
GIGABYTE GA-MA790XT-UD4P as an alternative to your current board. It already saves you $40. You can check Powercolor, ASUS, etc brands for GPU, and Corsair, Gskill, A-Data, etc brands for RAM. Again, I would not go with anything less than X4.