I would say it has already exploded. That doesn't mean it won't explode again.
Bitcoins? Don't mine those, at all. Seriously, not worth it.
Ethereum? Try scoring a used 290 from a reputable reseller on eBay. My personal recommendation is to run it at 900 MHz GPU and 1175 MHz RAM flashed to a 390 BIOS with voltage lowered as far as you can manage without instability (-100 mV is common; at that clockspeed, you might be able to do better, though you will see voltage use scale with clockspeed so be careful how you approach this matter). Use it to mine ZEC, then if you really want ETH (ZEC prices are also rising like mad), exchange your ZEC for ETH on an exchange like Poloniex. Then move the ETH off-exchange to a private wallet.
Or just mine ETH directly, though at that point you don't need the 390 BIOS so much. You still need the undervolt though. Do it right and you can get the card down to maybe 150W, which will let you use a 1KW PSU pretty easily rather than the 1250-1300W behemoths I got for my dedicated rigs last year. Also low power usage keeps the cards in service for longer, and can keep fan speeds low(er).
See if you like using the 290 in this fashion. If you do, get some more, put together a mining rig, and use your main PC to flash BIOS mods to the cards to prepare them for mining duty. Try to avoid reference cards where you can, but if you must use them, put them in a room where the noise won't bother you so much.
Lots of people out there don't seem to understand that Hawaii is still a pretty good card for mining, and it looks like eBay has a few for less that $200 that are NOT reference designs (which can be good or bad). Not great, not as good as the 480, but good enough. I mine with 8 Hawaii cards and one Pitcairn (only usable for ZEC) and I make a decent enough profit; more importantly, the ETH I mine today goes up in value tomorrow, so the reward for mining is much greater than any standard calculation will tell you. The only downside to my basic strategy is that I had to pay out-of-pocket for hardware, and I have to pay out-of-pocket for power. I can't/won't use ETH sales to "pay myself back". I haven't sold a single ETH yet.
If I had any room in my house for more mining rigs, I'd be buying myself more 290s and setting up one or two more.
Not sure if it's a bubble.
There ARE some rational limits to what people will pump into crypto - for example, my above tongue-in-cheek speculation that ETH could reach a per-token value of over $2 million is clearly ridiculous since the total market cap at that point would be over $206.9 trillion. The total household net worth of the United States is maybe a little over $96 trillion? Not gonna happen folks.
That aside, it's only a bubble if it's hollow. ETH isn't hollow, which is why it started getting attention. There's a lot of sidelined money out there right now looking for a good investment (one of the major causes of the 2008 housing bubble, and also one of the lingering conditions that hasn't changed since then). There are other reasons why I think people may be sheltering assets in crypto, but that goes beyond the scale and scope of this thread, methinks.
Still I do expect that a "crash" back to the $200-$250 range is possible. I just don't think it's inevitable.
The mining card situation is getting a little silly. I understand why people are snapping up all that hardware, but at the same time, it's hard for a gamer to get a decently-priced card just to . . . you know, play a video game. Instead you save maybe 3 months disposable income as a young, single guy/gal, buy a $1k video card, and then use it to mine to "pay for itself". Then after maybe 1-2 months you finally get to play some games with it.
Or you look at the lost income, realize that it's costing you something like $5 per day (or more) NOT to mine with it, and go back to mining since . . . those are some pretty expensive video games folks. Hell right now my 390 alone can pull in $7/day worth of ZEC. If I stop to play Fallout 4 or Dirty Bomb or . . . something else for 6 hours on a weekend, that just cost me $1.75. How about that?
I would personally be conflicted about playing anything but text MUDs if I didn't have multicard rigs elsewhere in the house doing the mining for me. The 390 is just icing on the cake. Icing that costs $.29 per hour to NOT use in mining.