If it's the SFF, there's no PSU upgrade. I just purchased the mini tower yesterday & it takes only the smaller ones, am going to replace with an EVGA G3 550W that's been sealed in the box for 2 years.
That's what I don't like about the DT & SFF versions, there's few aftermarket options. While I've heard of a 400W model (from some vendor in Texas) for the 780 DT, have also read it's not a drop in fit & not all required cables are there.
My suggestion would be to check out the models offered with the 990 SFF, One option would be the GT 1030, but only if the 2GB GDDR5 model, there's lots of GDDR4 variants, so watch out if pursuing that option. This is the one running in my wife's Optiplex DT, draws only 30W from the PCIe slot & also has HDMI 2.0 & Displayport 1.4, runs & looks good on a 4K monitor.
http://speccy.piriform.com/results/xlYujs3qWF8CC5cy1RCdgYn
However if gaming, best to use on a 1080p monitor.
Am hoping I can install the Dell OEM GTX 660 1.5 GDDR5 in mine upon arrival. May have to swap the SATA cables to the 90 degree type, as it'll cover these. May need to also get a UEFI BIOS for it, or run in CSR mode, the older a card gets, fewer will provide a link to the needed BIOS. That's what I needed to do for my MSI Radeon HD 7770 GHz edition, MSI sent me the UEFI upgrade link via PM.
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You'll find a solution by checking the fit before purchase. However, I cannot recommend any of the GT 730 versions. If there's a 1660 that'll fit & you want to purchase (can always use in another build later), I'd say that could be the best option. Just make sure it ships with both low and regular (tower) brackets. If by chance the tower bracket is installed, beware there's the possibility the card may require disassembly to swap the bracket. Another member has to swap one on a low prolile GTX 1060 to fit in a tower & said there was no way around taking it apart, having to replace the thermal paste in the process.
This (& the PSU situation) is why I never want another SFF or DT style PC, too much trouble at times finding what's needed & then the limit of what one can do.
However, you do have the option to install a SATA-3 SSD, that'll speed things up & also up to 16GB RAM. Good thing you didn't get the USFF, the RAM is cut to 8GB & I don't believe there's slot to install a GPU.
Good Luck!
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Cat