Not certain it will work - I dont know if it does support 800x600 - would be surprised if it didnt, but ....
If you go to MyDocuments - then My Games - then the civ IV directory, you will see the "CivilizationIV" configuration settings ini file. Take a backup copy, store in another directory so you can rollback without issues. Then open it up and change the values for Custom Screen Height in two places about 20 lines up from the bottom of the file. There are guide notes there - its straightforward
Be interested to know it worked ok
Regards
Zy