DDR3 is simply the ram type, it has nothing to do with the amount of channels. You could have single, dual, tri, or even quad channels with DDR3. (quad is the next one rumored to come out with the next Intel chips.) AMD and the newest i5/i7 chips use dual channel memory.
I wouldn't worry to much about the Dell PSU. They tend to use good ones and it can be replaced if you need more power.
Yes, I realized that after some further reading.
But do you know whether Dell XPS motherboard is capable of triple-channel?