425W should be more than adequate during booting and even just putzing around on the internet or reading mail, since most of today's cards don't consume that much power when idle. The test will be when you apply load to them playing games. In theory, a good quality 425W would be okay, but I doubt the PSU's that ship with Dells are top of the line.
I have a 570, which consumes more than a 560, and when playing games I rarely exceed 275W total for my entire i2500K system with three hard drives and an SSD (I use a Kill-A-Watt meter to tell how much I'm pulling from the outlet). I played Dead Space 2 recently and I don't think I went above 140 watts, and when idle, I hover around 88 watts.
Now, if I run stress both the GPU and CPU simultaneously using a benchmarking program like OCCT I hit around 450W, but that's not the kind of thing you'd normally see in the real world.
Decent 550W PSUs can be had for around $60-70 after rebate. The question is whether it'll fit in your case. I'm not sure how upgradable Dell cases are.
