What size and what type of power supply would I need?

I got an Optiplex 980 I am fidgeting with and it came with an i7 quad core 2.93 and 8gb of ram. The video card sucked as it was refurbished at some local shop. I swapped one of the 2gb chips out for a 4gb chip giving it 10gb of ram, and put a GTX 730 graphics card I got from a close out store for $4. It also has a 465gb hard drive. It has a 255W power supply in it that apparently was stock. My brother said it should handle it because the Dell power supplys are a lot better. However, I don't want it to short out. I tried to have my 400W put in it but it wouldn't match up. So what size and what type of power supply would I need? Note this is just a project, so please helpful answers.

pdl7562017-03-18T21:09:41Z

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Mini Tower or Desktop? It makes a BIG difference.
http://www.dell.com/us/en/business/desktops/optiplex-980/pd.aspx?refid=optiplex-980&cs=04&s=bsd


Edit-
Now I understand what you meant when you said the 400W power supply "wouldn't match up". That systems motherboard uses a 24 pin Micro-Power connector. That system can use a standard ATX power supply, but you'll need an adapter for the 24 pin connector. It looks like this-
https://www.directron.com/hp24mini.html?gsear=1
Make sure you install the GT 730 into the Blue PCI-E x16 slot...the Black PCI-E x16 slot is wired x4.
http://sites.fas.harvard.edu/~phys191r/Bench_Notes/optiplex-980-tech-guide.pdf

m8xpayne2017-03-18T19:28:38Z

The GT 730 only consumes 30w to 40w of power and that's when you're playing games or watching videos. You should be able to boot up the system with the stock power supply. If all parts were up and running at full load then it would come close to the maximum capacity of the stock power supply and that's when you're running a risk of the system shutting down .

Pretty much any Power supply you can buy online follows the ATX form standard so they are all the same. The problems with finding a new power supply arise.It seems like you have the flat desktop version so you would need to figure out the form factor of that power supply then find one that matches.

If the motherboard is a standard sized mATX board, you can always find a cheap ATX case then transfer all the parts to that case. Then you would be able to use a standard sized power supply. Dell has a nasty habit of using proprietary parts so you'd have to verify they you have a mATX motherboard.

Fred2017-03-18T19:25:41Z

Dell is famous for using non-stock parts such as power supplies. You may have to buy a power supply from them or do some handy work of your own to attempt to fit an ATX power supply into the Optiplex case.

Spock (rhp)2017-03-18T19:06:02Z

psu sizer app online. i use the one on the lower left of newegg's power supplies page