Well that's what's smart about Nvidia. Unlike AMD, they don't allow SLI with different cards, so your option would be either get 1070 for $350 or so which will be as strong as the 980ti (let's say), so it won't be an upgrade, or pay a premium price for a second 980ti so you can go SLI. Unlike nvidia, you can crossfire the 290 with a 390.