There are huge amounts of hardware for this. Start at newegg.com for the best deals
You can start in motherboards
http://www.newegg.com/Store/Category.aspx?Category=20&name=MotherboardsOr motherboard/cpu/vgacombos:
http://www.newegg.com/Store/SubCategory.aspx?SubCategory=446&name=Motherboard-CPU-VGA-ComboCases
http://www.newegg.com/Store/Category.aspx?Category=9&name=Computer-CasesMicro cases:
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=2010090007%201054808292&name=MicroATX%20DesktopYou'll find mini cubes with mini motherboards there, you just add on your ram/cpu/hard drive.
I'd say 95% of all modern motherboards have video already integrated on the motherboard, ranging from intel, ati, nvidia graphics that will suffice your needs.
All modern motherboards you can run server 2003, the key is to make sure there are win2003 drivers for it, and again, anything modern you can be assured there is.
You'll find very good combo deals there, there's so much hardware to run server 2003, it's nearly impossible to make a recommendation. Find something within your budget.
You can buy a decent used pc as well and load 2003 on it. But for about less than 200 you can get something new with no monitor.