I remember 768 bit asymetric keys in the mid 90's, and the paranoid had perhaps a 1024 bit key (I'm not very paranoid, and I got a 2048 bit key before 2000). A 768 bit RSA key was factored in 2009; the NSA could probably do it earlier. The default now is 2048 bits, and the key used by Wired is 4096 bits.
That's a lot of doublings of the difficulty to brute force a key. 2^3328 increase in difficulty.
RSA factoring does not increase with keysize increases like you might expect. Factoring, while still very hard, is much better than brute-force and continues to see improvements.
That's a lot of doublings of the difficulty to brute force a key. 2^3328 increase in difficulty.