I play a lot of games and would like to run things like Skyrim and BF3, if it's possible. I also do video editing and things of the sort, so I do a bit of everything. Also, a video card with at least two DVI ports would be great.
The PSU I currently use is not sufficient enough for anything (I had to replace mines with one from an old Dell.)
Edit: So I went researching and was gonna go for this CPU/CPU Cooler setup:
Intel Core2Quad Q6600 w/Hyper Master 212 Evo
First of all, you done good search - maximum CPU upgrades are Q6600 and Q6700 (too expensive for nothing).
With Q6600 you can use BSEL tape mod and overclcok (if you have locked BIOS, certain XPS models had overclock option in BIOS, do you), this will increase you CPU speed to 3GHz, will help you with this if help needed.
CPU Cooler, you really know that you have BTX case, so you have BTX cooler as well, you did not know?
There are 2 options here aluminum (junk) and copper (better)
If you have aluminum you have to upgrade.\
Next, GPU. I would stay away from Gigabyte due to oversized fans - you have GPU holder (blue panel closing on top) would it close properly?
Next PSU. You can place any standard ATX PSU.
Check the picture below for the proof of ATX PSU, on the same picture you will see Aluminum CPU cooler as well, so you know what you have to identify.