It is compatible and it will support the video card, but unfortunely the PSU isn't enough you will need to replace it since 375 W is not enough. And also the cpu is very low end and will bottleneck the video card, so you will be running it with bad performance in a lot of games only because your cpu is not good enough.
Minimum specs for you to buy it, is replacing PSU i don't think your psu will handle the video card properly.
Then it will be working but you will not be fully enjoying the 7970 performance, so it is recommended replacing CPU by doing so you will need to replace motherboard,memory,etc... basically i would recommend you buying a new computer if you are thinking in playing with a high end video card like the 7970
What processor are you running? I'd agree that a 7970 is overkill, but upgrading your power supply and getting a new video card is probably the most bang for your upgrade buck right now. A couple years ago I had an XPS 420 (same case I think) and replaced the psu with a corsair tx650w (about $70-100) and got an HD 5850 (when that card was the current gen). You should look consider something similar but shoot for a GTX 560 ti- there are some great deals right now.
DDR5 is used in video cards. DDR3 is used in your systems RAM.
Make sure if you do a new build that you plan on getting a new case- standard size motherboards typicall dont fit Dell cases (they're "special" like that).