TylWhite :
I play all the latest games coming out, as my budget pc was a test to see if i liked gaming on the pc and i love it and will never go back to primarily console.
Like eggz said up above. It's tough to say...The plus for the 390 crossfire is that you get better performance by higher fps in everything now. Honestly you probably wouldn't have to worry about 1080P getting below 100FPS for quite awhile.
Now the drawbacks...it's an amd card so even without thinking about crossfire issues, you will have issues in certain games like witcher3, grid autosport, project cars, GTA V and any new games that don't work well with amd...there's a couple other ones that just don't work well with AMD cards.
Now, crossfire...they are getting better and now that win 10 is out and dx 12 is supposed to help with sli and crossfire but we need to see it to believe it, and in the meantime there will be issues just being in crossfire...good profiles for new games, etc.
Power consumption and heat....definitely something to think about.
If you absolutely want the better performance for $650 right now...that would be the way to go, just remember the drawbacks. However in 2 or 3 years when you go to upgrade, you're stuck with the 390's and will have to spring for a top end card, while with the 980ti, the prices should be much lower and you could sli at that point...which would destroy the 390 crossfire and keep you gaming for another few years without full upgrade.
Maybe you'll want to upgrade anyways in a few years for HBM and the 16nm nvidia or amd cards because they will be more powerful than the 28nm we have now. In that case, if you can live with the drawbacks crossfire away and enjoy it.