How do I know exactly what motherboard I have in my comp?

CPU-Z is a freeware that gathers information on some of the main devices of your system.

CPU

* Name and number.
* Core stepping and process.
* Package.
* Core voltage.
* Internal and external clocks, clock multiplier.
* Supported instructions sets.
* Cache information.

Mainboard

* Vendor, model and revision.
* BIOS model and date.
* Chipset (northbridge and southbridge) and sensor.
* Graphic interface.

Memory

* Frequency and timings.
* Module(s) specification using SPD (Serial Presence Detect) : vendor, serial number, timings table.

System

* Windows and DirectX version.

Cpu-z, sisoft sandra, everest, pc-check, a lot of open source programs I am sure from sourceforge.net, some I can't remember if they are still out there.

You can also check your manufacturer support page with your dell service tag or serial number.

If you don't believe any of them, you can open the case and read it off the motherboard or run the FCC ID Number.