Please list your pc components so we can make a valid recommendation. Without knowing your actual components it's a crapshoot!
Of the two power supplies you mentioned, the Corsair VX550 is the better choice. Corsair has a reputation for high quality power supplies, They consistently earn high marks in technical reviews. They are stable, reliable, and come with at least a 5 year and depending on the model sometimes a 7 year warranty.
The general rule of thumb is a high quality 500 to 550 watt power supply with suffcient current (amps) on the +12 volt rail(s) can easily power a system with any single video card made.
Now, since you have an off the shelf system it's quite possible your pc might not be able to keep up with the latest and greatest video card.
From the physical dimensions of your micro tower case it appears as if any standard atx power supply will fit. The problem with Dell and other off the shelf brands is that they do not provide very much information about the power supplies they use.
I would suggest you take a look at either Corsair 650TX or Antec EarthWatts EA650. I personally would go more toward the Corsair, but that's just me. The Antec has a 6pins PCI-E connector plus a 6+2pins one; the Corsair has 2x 6+2pins.