For a real basic computer (No high end games)
I would go for:
All in one motherboard (You all forgot that..And its better than buying PCI Cards like your talking about, because they become expensive)
At least a 40gb Hard Drive
At least 512mb RAM Although you could use 128 if you really need to save money
A decent processor (Id go for a Celeron D)
A CD Drive
A Floppy Drive (If needed)
OS: Whatever you need, Newest would be best but if your comfortable with 2000 and whatnot you could use that
As for your Excel Question.. Look on the box.. System specs