Well, the thing is that my power suply is some special model .
Dells own :? (I have Dell XPS) so i don´t think i can just buy a power suply and change it.
You're right: Dell's PSU's and mobos are proprietary, meaning that you won't be able to easily use 3rd-party components. If you ever want to upgrade your PSU, you don't have many options. There's only one that I know of: PC Power and Cooling. You can buy direct from their website, though I have seen a few on sale through other vendors.
Be warned: PCP&C PSU's are probably some of the most expensive PSU's on the consumer-level market, but they're also cosidered one of the best PSU manufacturers. They have a list of Dell PC's and compatible PSU's, but the list might be slightly outdated (the XPS PC's were paired with PSU's lower than your current PSU). You should probably call them to see what they recommened.