for mATX that can do 800x480 with onbaord grpahics you will most liekly need an nForce 2 IGP board. If you want to stick with the P4 you will need a separate AGP card. i would sll the P4 and get a mobile athlon and an nForce 2 but that is jsut my opinion. There are other options. The sempron would be cheaper and easier to build though unless you get a Biostar mobo (due to CPU settings with the mobile athlon but this wont work with old Opus PSUs so you need to check that). Do some more reading about the mobile chips and mATX setups ont his site.
Welcome by the way. Boris will be glad to see another Lexus.