As an alternative to the current mechanic of researching tech, and then getting a breakthrough (of which you choose one), I was thinking of the mechanic demonstrated in spell research, where you build up research points, and then purchase your upgrade.

In research, however, prior technologies may need to be present for new technologies to show, or prerequisite technologies.  Therefore, purchasing a tech won't be such that you can save up your research points to buy an ubertech; it will have prerequisites to it as well that must be researched first.  But, you can save up research points to buy your way down a tech tree pretty deeply if you find you suddenly have a need.  Imagine in GCII if you could light up Lasers 1-5 in one shot by saving up your research points and then loosing them all in a shot; you can leapfrog someone in a defensive technology, and cause them to change tactics.

Anyway, food for thought; since we're kind of doing this anyway, it's just you have to make a decision up front.  Maybe you can choose which of the 5 tech trees you dump your research into (or have sliders to allocate different percentages into each of the 5 trees) and open a tech tree and buy the research when you've accumulated enough points.

I don't imagine there should be an upper limit; you can just keep stockpiling research points, so you can get multiple research advances in one turn in a particular tech tree, if you've gotten enough points and have the prerequisites.  Or you can buy the prerequisites as well.

Winni