I bought an XPS 435 MT with the low end card and a plan to upgrade it myself. Then I bought a swanky BFG GTX285 with 2GB. Time to upgrade the power supply - things are a bit tight, so I went with a Corsair HX650W.
After install, the light on the MOBO shows, but pressing the power button does nothing other than cause it to blink off then back on once. I reseat and retry with same result. So I take my sys to the local computer store where a friend double checks me by plugging in his Corsair 650. Same result. I plug the factory 350W back in, all is well. He plugs in one of the store's 350's and all is well. Back in with my 650: MOBO led on, blinks off/back on when power button is pushed, no fans, no post, nada.
Corsair and Dell have had compatibility issues since the XPS 420, best thing to do is return the Corsair and replace with another brand.
I know that the Antec EA650 worked but it doesn't quite match the quality of the Corsair HX so I'll reccomend the Antec TPN 650.
No, I'm not an Antec fanboi, the Corsair HX620 worked perfectly in my XPS 410.