I got a Dell XPS 8300 as a gift while back and I want to replace the graphics card, but I may need a new power supply. At the moment, I have a 460w that came with it, but I want to replace my gtx 560 ti oem with a gtx 660 ti.
Think the real issue will be of the PSU will have the necessary connectors than if it has enough wattage.
But yea, I would replace the PSU. The PSU's put in prebuilt systems are never that great and PSU's degrade over time (though I suspect from the CPU its under 2yrs old, so wouldnt have degraded too badly)
You can almost guarantee that the Corsair PSU will also provide more stable, cleaner power with better longevity than that Dell PSU and that's just as important as wattage. A lot of cheap PSUs advertise higher wattage than can actually be delivered and while the 660ti isn't a high draw card may over draw the Dell PSU even if it has the connector.