I think it's easier to deal with an isolated start in Civ4 than in previous iterations of the game:
1. As a previous poster said, you can focus on infrastructure rather than military since the only threat is barbs (I've often not bothered to research any military techs besdies Archery until I've made contact).
2. The research cost of techs decreases the more civs that know them irrespective of whether you've met them or not.
3. You can't trade techs until Alphabet, which the AI doesn't seem to prioritise, and which isn't too far from Caravels anyway.
So, I wouldn't worry too much about being under-teched, or at least this isn't a problem I've come across much. What is tricky is having to work with a much more restricted set of options: no-one to trade resources with, no-one to attack, and usually a restricted set of resources.
In this case I generally focus on fast expansion and infrastructure, targetting techs and/or wonders that give me some advantage above the starting traits, e.g. Colossus on archipelago or continents where I'm likely to have many coastal cities. Or Metal Casting for forges if I have several metals resources to get more happiness.
The overall goal should be to get off the isolated start - so focus on that target and that target only until i) you've completely settled your start and ii) you've got caravels.
Once you've finally met come other civs you can gauge your relative tech position (this is often very interesting!) and hopefully identify some nice places to settle with a view to getting a beachead for future military operations.
