Because he would probably also need an new PSU and maybe even mobo. he already has a card so he needs 3 Pcie 16x slots (at least 16x size, not speed).
Mining is fine to pay off a card you own anyway. Else if you want to make bank, just invest some money ($1k+) and wait.
No he would just need a few powered 1X-16X risers. They work fine in most 1X PCIe slots. Sometimes you need to make a "dummy plug" to get Windows to recognize the cards but you can make or buy these for cheap. I agree if he needs a new power supply it's not really worth it. A good 850W power supply with at least 75 amps on the +12 should easily handle two 390's comfortably but three could be pushing it even with undervolting. Topping out a 850W with a 4GB 380 may be a smarter buy as it saves from having to buy a 1KW supply, and 380's use a lot less energy under loads. An extra 20Mh would net a total of around 80Mh machine for 850W. Not bad.
If you're serious and really believe in the future of Ethereum the best approach is both mining and buying IMO. The mining part is dependent on many factors (cost of electricity, cost of hardware, warm or cold climate, noise pollution etc.) where the purchase of Ethereum is a much more straight forward endeavor. However all of our situations are unique so you need to assess your own factors and decide.
Mining is still very much worth it if the above factors play in your favour but there's always a time limit on mining profitability due to a number of factors which have been discussed Ad nauseam and not worth repeating.
I personally bought as much Ether as I plan on mining in total (so around 50/50 split). If Ethereum tanks in price your investment in simply buying Ethereum may lose you more money compared to buying a few mining machines which costs can be recouped from reselling hardware. On the flip side all efforts towards mining will seem silly compared to buying (at today's price) if the price of Ethereum shoots up to 50 USD a coin.
Last edited: