Let's see... 150,000,000 keys per sec on a 7970, or 1.5 * 10^8 k/s. There are maybe 1.0 * 10^8 Bitcoin addresses with unspent outputs.
If I remember right, the complexity of addresses is 2^128. So, you have 1/(1.0 * 10^8 2^128) = 3.4 * 10^30 keys as the inverse of your probability for finding any private key to some unspent output (or basically, your reduced search space after accounting for all keys with unspent outputs).
That's (3.4 * 10^30 k [1.5 * 10^8 k/s]) = 2.3 * 10^22 seconds to find a single private key to spend a random address' Bitcoins with a single 7970.
That's only 52,000 * the age of the entire universe, so you should find one pretty quick!