However, I got to wondering about the security of PGP assuming somebody trying to read my PGPed stuff has my 1024-bit secret key. ie, if I have it on my personal computer, and somebody gets my secret key, how much less robust has PGP just become, and what are appropriate and reasonable steps to take to protect this weakness?

The security of the PGP system is based around the security of the PGP private (secret) key. The security of the secret key is based on two things: 1) The bits in the secring, and 2) the user pass phrase. An attacker needs both 1 and 2 to compromise a secret key. Posession of only one of them is not enough. It doesn't matter how big your key is. It could be 384 bits, it could be 2048 bits. You still have the same two walls protecting your key. If someone has read your secring file, that means they only have to get your pass phrase. This gives an attacker something to go for. Once they obtain your pass phrase as well, your key is compromised. If, however, they obtain your secret keyring but they CANNOT obtain your pass phrase, you are still safe. It just means you have one fewer walls surrounding your secret key. I always assume that someone has compromised my secret keyring, in that someone has obtained a copy of it. So I spend most of my "time" (granted, it's not a lot) trying to protect my pass phrase. I have a fairly long pass phrase, more than 40 characters, and I make sure I KNOW (not believe, KNOW) the path the bits take from the keyboard to the CPU -- and I make sure that path is secure. Otherwise I don't run PGP at that time. Since I have a laptop, that makes my life much easier -- I cut-and-paste my PGP mail over to the laptop and run PGP there.

From the laptop I can send mail out directly.

I hope this answers your question, -derek