OK, this was not easy, but I got it. Yes it can be upgraded, however in the first choice it is 400 watt of unknown manufacturer udapted to fit GX620 and plenty more similar Dell PCs
Here we go, first choice at Amazon you can get this PSU with all connectors, I think http://www.amazon.com/ATXPowerSupplies-com-HP-Q2828F3P-...
Or you can get directly from manufaturer website, in this case you can choose which connectors you need, there is drop down menu where you can choose components, look a bit complicated, but I think it is better than Amazon http://www.atxpowersupplies.com/350-Watt-Dell-F5114-Pow...
So, what is going on above - standard PSU is fitted with metal plate which has mounting holes adapted to be used in GX620.
Now, I proceed to choice 2, adapting regular ATX PSU to GX620, this is hack job - you would have to do some modifications to fit this PSU, reward is 500 watt, maybe stronger can be used as well from the same PSU family line.
One person adapted Antec earthwatts EA500 to GX620 those are the links, first newegg, which is disactivated, but it contains good specs http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Next is Amazon, here I provide all possible choices, in case if one will be sold out, the other one will stay http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias...
I just completed the power supply upgrade using an Anted EA-500. [he meant Antec]
The purpose was to provide power and especially the 6 pin molex connector for an nVidia Quadro FX 3450 graphics card. The Optiplex GX620 Mini Tower is up and running with no serious problems. However, I had to make two modifications to allow the new PSU to fit.
1. The chassis has three metal tabs that are designed to hook into and stabilize the Dell PSU. When the locking tab is depressed and the PSU slid away from the opening about an inch, these disengage. Since the new power supply had no slots, the least destructive and quickest solution seemed to be just to bend the tabs back out of the way. I accomplished that by pushing hard with a screwdriver. If need be, they could be returned to position.
2. One end of the power cord receptacle on the new PSU hit the edge of the chassis opening on the back of the computer. In that position, it would have been impossible to refasten most of the screws. I was able to remove one screw from the power recep. and cut off the plastic ear that was in the way.
With those changes, the power supply bolted up securely with all four screws and fit acceptably.
The SATA power cables are right angle.
You may need to get a Right Angle SATA Power connector for the hard drives.
Otherwise you can shear them off closing the cover.
That was very extensive coverage. Alain123 is there anything left you did not ask question yet? Sometimes it is difficult to find your questions, I can not say that I can answer all of them, but if it is related to Dell upgrade, please PM me a link to your post - this way it would be easier for me to help you.