I've got a 110 yr old farmhouse with original 3.5" wide hardwood floor nailed dierectly to the floor joists. No subfloor. I want to put in a new hardwood floor. Can I go directly over the old hardwood with new? Thus making the old hardwood floor the "new" subfloor?

Yes you can...

Can I put directly over the old hardwood or do I need something between the new and old. I've been told I need 1/4 plywood in between the new and old. Id like to run the new in the same direction as the old. Thanks.

If you want to go the same direction you will need to put a layer in between. I wouldn't go with less then 3/8.

Thanks for the tip. What if I go perpendicular - would I still need something in between?

Yes you can go perpendicular...guideline would dictate to still provide an additional layer,,but my experience would tell you its ok..

Thanks James,
I think I'll go perpendicular without the layer since I want to keep as much ceiling height as possible.