Bottom line (most likely) is that you will need to buy a new case (with power supply) for your new mobo.
When I was planning on upgrading a Dell XPS T500, I found out that Dell uses modified oem motherboard with proprietary power supply connectors and case connectors. Although it is a different model than yours, I think some of the problems will be the same.
One option was to have PC Power and Cooling modify a power supply to connect to the Dell oem motherboard. The problem there was that you would not be able to use that power supply with non-Dell mobos, e.g., if you wanted to upgrade in the future.
If you want to change your motherboard, the Dell power supply connectors (yes two) will not connect properly to your new mobo, unless you get a Dell motherboard. Also, the connectors from your Dell case (power switch, leds, etc.) will most likely not conform to the new mobo. As a result, you would need a new case and power supply for your new mobo.
When I first bought the Dell, I bought it because I thought it would be easily upgradable. Well, some things are easy to upgrade, like sound card, video card, etc., but more significant changes, like a new motherboard, meant additional purchases. In the end, I bought a new motherboard (s1854) and case (In-Win q500 with 300 watt ps) and used the hard drive, nic, sound card, floppy, etc. in the new pc.
If you do buy a new case, you will have a much more easily upgradable computer, which is nice. Good luck!