For what I saw, you're using an implementation of the Sieve of Eratosthenes, which should be one of the most efficient ones for numbers up to 2^33. If you want to improve it, perhaps you should look into the Sieve of Atkin for O(N/log log N) operations instead of O(N).

Larger primes use different methods to prove primality but if you can't control your input (I mean, if you're not just trying to find a large prime but rather to determine given a number n, if it's prime or not), then you'll most likely end up with some complex algorithms such as Elliptic Curves or AKS.

Sorry for not posting any code, though :-(

Cheers,
-b