2x r9 290 Crossfire vs Rx 480

1) Power draw and heat
2) Screen tearing, stuttering, multi GPU latency issues
3) Lack of support in some games, maybe even worse performance than a single card.

I personally feel like multi-GPUs are something that should be left in the past back when a single GPU wouldn't cut it for high end gamers. Some people's experiences vary, but to most, it feels like it's a chore. I don't have any CF/SLI setups of recent days (my newest is a 660ti 2-way) to prove this for new games, but I believe your best option would be to sell the 290 and invest more into a GTX 1070 for better single card performance and game away at the same performance level 2x 290's would give you in a real world scenario. You'll also draw less power and run cooler, and depending on how long it lasts, you might pay off the difference between a second 290 by saving on electricity.

In terms of raw performance the R9 290's on crossfire are better, hands down. You will have more FPS, and better benchmarks.

That said all the drawbacks that Justiceinacan mentioned are very real. In fact I dumped my 2 R9 280's in crossfire for an RX 480. While it benchmarks slower, it "performs better" in that it works in every game, there no microstutter, no tuning on a per game basis that needs to be done, and not 2 monster GPUs filling the room with heat.

drawbacks:
1. some games don't support crossfire, although a majority do.
2. r9 290 uses double the wattage (300w vs 150w) so two of the r9-290 would be 600 watts. you would need quite the power supply for it.
3. the performance between the r9-290 and the 480 are so close that if you are to switch it might be pointless or insignificant of a difference.

bonuses:
1. you'd save wattage/power by switching to the 480.

recommendation:
1. keep the r9-290 and crossfire it if you wish for more power today but ONLY if you have a good power supply.
2. save your money, don't crossfire and don't trade for the 480. wait for Vega then decide.


edit: i recently changed my 660Ti SLI set up to a 1060 and the performance was about the same. a very small difference. by switching the r9-290 to the 480, you may see the same result as my experience: a disappointment for the money spent. so i returned the 1060 and got a 1070.
i'd say hold onto your current card and see what you think of AMD's Vega coming this year.

Over 80% of our builds up thru the 9xx series have been SLI or SLI ready.... have yet to observe microstutter, screen tearing, etc and w/ scaling at 70% on average including games that didn't support SLI far better return on fps / dollar, Here's the results for the 9xx series based upon testing at techpowerup

Dragon Age Inquisition* goes from 38.8 fps with a 980 to 66.0 with twin 970s for performance increase of 70.10%
Thief goes from 81.1 fps with a 980 to 136.1 with twin 970s for performance increase of 67.82%
Far Cry 3 goes from 41.0 fps with a 980 to 68.8 with twin 970s for performance increase of 67.80%
Tomb Raider goes from 35.0 fps with a 980 to 58.7 with twin 970s for performance increase of 67.71%
Battlefield 3 goes from 73.1 fps with a 980 to 121.4 with twin 970s for performance increase of 66.07%
Bioshock Infinite goes from 88.0 fps with a 980 to 143.9 with twin 970s for performance increase of 63.52%
Crysis 3 goes from 26.8 fps with a 980 to 43.3 with twin 970s for performance increase of 61.57%
Splinter Cell: Blacklist goes from 57.1 fps with a 980 to 92.2 with twin 970s for performance increase of 61.47%
Battlefield 4 goes from 53.1 fps with a 980 to 83.2 with twin 970s for performance increase of 56.69%
Batman: Arkham Origins goes from 95.0 fps with a 980 to 148.3 with twin 970s for performance increase of 56.11%
Assassins Creed* Unity goes from 24.1 fps with a 980 to 37.0 with twin 970s for performance increase of 53.53%
Far Cry4* goes from 53.4 fps with a 980 to 81.7 with twin 970s for performance increase of 53.00%
Shadow of Mordor* goes from 70.8 fps with a 980 to 107.1 with twin 970s for performance increase of 51.27%
Ryse* goes from 62.4 fps with a 980 to 94.3 with twin 970s for performance increase of 51.12%
Watch Dogs goes from 57.1 fps with a 980 to 85.1 with twin 970s for performance increase of 49.04%
Grid 2 goes from 95.5 fps with a 980 to 141.8 with twin 970s for performance increase of 48.48%
Assassins Creed goes from 43.2 fps with a 980 to 61.8 with twin 970s for performance increase of 43.06%
Crysis goes from 52.0 fps with a 980 to 74.0 with twin 970s for performance increase of 42.31%
Civilization Beyond Earth* goes from 80.2 fps with a 980 to 113.7 with twin 970s for performance increase of 41.77%
Metro LL goes from 53.2 fps with a 980 to 74.6 with twin 970s for performance increase of 40.23%
WoW: Mists of Pandaria goes from 126.3 fps with a 980 to 174.8 with twin 970s for performance increase of 38.40%
COD Advanced Warfare* goes from 113.1 fps with a 980 to 154.5 with twin 970s for performance increase of 36.60%
Witcher 3 goes from 48.0 fps with a 980 to 63.8 with twin 970s for performance increase of 32.92%
Alien Isolation* goes from 99.0 fps with a 980 to 121.0 with twin 970s for performance increase of 22.22%
Diablo III: Reaper of Souls goes from 183.0 fps with a 980 to 220.8 with twin 970s for performance increase of 20.66%
Wolfenstein: New Order goes from 61.8 fps with a 980 to 56.2 with twin 970s for performance increase of -9.06%
Dead Rising* goes from 43.6 fps with a 980 to 38.6 with twin 970s for performance increase of -11.47%

That being said, we have had little experience of late with CF. While I won't knock CF in and of itself, it is generally accepted that SLI game support is better than CF. OTOH, in the price / performance niche that I would use SLI or CF, there's hasn't been a AMD card that really "measured" up the equivalent nVidia card is quite a while. While the AMD 3xx series cards were overclocking in single digits, he 9xx series was 25 - 31%. For 10xx however, haven't seen a reason to pursue dual cards, at least up to1440p.

Now for your particular case, if you were asking this question last generation., I wuda said from a performance standpoint, the CF option is probably most cost effective in and of itself. However, when considering the extra expense PSU and extra fan wise due to all the heat and wattage required.... that option gets less attractive.

However, the AMD RX 570/580 is supposed to drop on 18th April, 2017. Even if you don't choose one of those, it will knock down the prices on the 480.

If we are talking "what should I buy today?", I'd have to say the 1060 ... but again those will be cheaper come mid April after the 5xx series comes out.