Ive just picked up 2 360's that need a bit of attention, both have Dashboard 2.0.6717.0. Is it worth keeping them at that version or should I update to the newest one??

Keep them at the dashboard that they are at now. They can be JTAG'd. If you don't know what that is, then do a quick Google search or I will go more in depth if you would like to know more.

I thought that would be the case. I knew newer dashboards couldn't be JTAG'd but wasn't 100% sure. I will leave then as they are then.
Will leaving the old dash make them better sellers? When I finally get round to selling them that is.

For the right buyer it would be worth more. Could JTAG them yourself and make even more on selling them.

^^This. They will sell better than a normal console if you list them at JTAGable. If you actually JTAG them, though, then they will sell for a LOT more depending on the condition they are in and what motherboards they have.

They are both Xenon. At the moment they are rough. One has the CD tray, front and ends missing the other is just scruffy. Ive got plenty of bits to tart them up. Both will have Hybrid X clamps and extra heat sinks on the ram, SB and ANA. I dont like to sell tat not unless it is sold as tat