HI Norbimu, 

Thanks for writing to us. 

255W is the maximum and compatible threshold test by Dell and can be recommended.

There are chances a higher wattage PSU would work but not advisable as you might risk damaging the motherboard or any components

Regards 

Hello! Thanks for the answer!

Another question if I may ask...

Can I use this 255 w psu with any dedicated vga cards with low tdp, like nvidia gt 610 (~30w tdp) or radeon hd 5450(~20w tdp)?

Current 780 spec:

dell f255e-00 255w psu

Core2duo e8400 (65w)
intel q43/q45 with gma 4500
Hitachi 160gb 7200rpm sata
2x2 gb ddr3 ram
Usb keyboard, and mouse

(dvd rw- disconnected from psu, I don't use it)
bios v. :a15

5450 works.

The other option is to use a regular power supply outside the case with the hole left by the internal supply as a way to get the wires inside.