There are some obvious holes in your strategy.
If your PC catches a virus and the anti virus software detects it and removes it then it is irrelevant whether your drives are external or internal because no harm is done. But if the a/v software doesn't detect it then the chances are you won't know that it is there and will therefore merrily connect your hard drives and expose them to it.
Also whilst it is easy to have data on external drives i.e. music, programs such as games are more problematic. All programs need to create entries in the Windows registry so expect to be installed in the same drive/partition. theoretically you could have the bulk of the program on another partition but in practice most programs either won't let you do this or you take a performance hit (certainly if the drive is external).
The cleanest installation is always to have Windows & programs on one partition and data in another (or several). That way you can reinstall Windows & programs without disturbing your data.
I also don't see the point in making things like sound cards external. Most of this stuff is now integrated on the motherboard and I would have thought that would be the ideal if you want to go small.
Have a look at itx based systems & components:
http://www.itx-warehouse.co.uk/productlist.aspx?Cid=42