Many people on Less Wrong believe reducing existential risk is one of the most important causes. Most arguments to this effect point out the horrible consequences: everyone now living would die (or face something even worse). The situation becomes even worse if we also consider future generations. Such an argument, as spelt out in Nick Bostrom's latest paper on the topic, for instance, should strike many consequentialists as persuading. But of course, not everyone's a consequentialist, and on other approaches it's far from obvious that existential risk should come out on top. Might it be worth to spend some more time investigating arguments for existential risk reduction that don't presuppose consequentialism? Of course, "non-consequentialism" is a very diverse category, and I'd be surprised if there were a single argument that covered all its members.
Hmm, that's a tough call. I note however that at that point, where your marginal dollar goes is more a matter of a cost-benefit calculation than a real difference in preferences (I also mostly care about currently existing people).
The question is, which will maximize life expectancy ? If you estimate that existential risks are sufficiently near and high, you would reduce them. If they are sufficiently far and low, you'd go for life extension first.
I reckon It depends on a range of personal factors, not least of which your own age. You may very well estimate that if you where not egoist, you'd go for existential risks, but maximizing your own life expectancy calls for life extension. Even then, that shouldn't be a big problem for altruists. Because at that point, you're doing good for everyone anyway.