If you are going to video edit seriously, check your software documentation and the vendor/publisher website and see what GPU acceleration it can use and which it responds to best.
For gaming the R9 290 and GTX780 run vey close and both are available with excellent (quiet and cool) third party heatsinks, but be aware some of the AMD cards (Sapphire TriX particularly) are very long, so check the case you finally decide on can handle one.
Agree on the memory, at least try the system with 8Gb, if it's good enough you've saved enough to give yourself a little treat, if not, well, it's not exactly hard to install is it? 😉
If this is going to be used for REALLY serious stuff, don't forget to add a second HDD for backup, or to run in a...