I don't think so. I just read somewhere hear today that your warranty is void if you buy a GPU from another vendor. I would assume that they wouldn't support a more powerful PSU for another company's GPU.
Can you perhaps place the PSU above your optical drive or somewhere else?
The power cable for my CPU wasn't long enough so I placed my PSU outside of my case with the side panel open for a week until I could get an extension. Maybe Dell owners could do that on a permanent basis.
they prolly good, but u would probably be better drilling a hole in the side panel for the cables, and i would be a little uncomfortable having a case permanantly open. Heat and dust.