Marko,
The biggest difference is tha additional channel and the diode to protect from a spike. The data sheets do point out exactly what the diff is, but they are essentially both Darlington arrays ganged together in a Dip chip.
Wheelchair Bob
A 2803 is eight darlingtons with common emiters and is dip8.
A 2003 is 16 bit 2x8x interpolating Dual channel DAC with intergrated PLL. It says here,
Oh and is a Dip16.
Bazzer
Bazzer kind of has it wrong.
I still maintain you should get the real scoop on these devives by reading the data sheets. GOOGLE will be your friend if you let him.
These were taken from the data sheets via Farnells links.
If you'r that good at making things easy, take a look at my threads, Just a Quickie & Design problems, I'm getting no where with them.