It doesn't look like the Optiplex 790 is SLI or Crossfire capable. And even if it was, your CPU would be a bottleneck I'm sure.
If you need to figure out watt requirements use a good online PSU calculator like this one:
Power Supply Calculator - PSU Calculator | OuterVision
Make sure you select SLI.
Name brand PSUs like Cooler Master, Corsair, Thermaltake, or Antec make a decent PSU. I prefer Antec myself. I've owned two and none have failed me yet. You might want to go modular so that you only use the power cables you need to keep the inside of the case clean and neat. But it's not a requirement. You'll often see PSUs rated with a metallic rating like gold or platinum, etc. I've read there's very little difference between those ratings and their so-called efficiency status. Just like EnergyStar, but that's a whole other subject. And I'm sure anything that advertises "green this or that" crap is just another marketing ploy.
Replace your PSU at least every 7 years. Good quality models would probably last at least 10 years and that's what a lot of good PSU manufacturers are offering as a warranty period. A PSU is the heart of the system and if it goes out it could take the whole computer with it. I'd also invest in a good UPS so that a power failure doesn't mess with the Comp. I own
this particular model, and I chose that one after asking the company if it would be compatible with an active PSU. Which it is. Many UPSs aren't active PSU compatible. So don't make that mistake and shoot the company an email to be sure.
Don't make the mistake of getting a 1000 watt beast for something that only needs about 500 watts. Many people do this. Newegg.com usually lists the watt requirement of a GPU, so you can plug that into the PSU calculator.
If you really want to go SLI, I'd build a cheap Comp. that allows it. Check out pcpartmaker.com and focus on a micro motherboard for cost savings. For your crunching I'm sure you could get by with LGA 1151 and Windows 7. Like an i5 6600k or i7 6700k. I bought my i5 on eBay for around $190 if you can believe it. Since it's rare that there are issues with CPUs, an eBay CPU purchases shouldn't be all that risky. But read the seller feedback, and the auction description well. YMMV. Motherboards on the other hand can be iffy and I would be hesitant on buying an used MOBO on eBay. Unless it's brand new in the box.