Goodwill or hard pressure?

As I mentioned in the other thread I started here ( http://consumers.creditnet.com/straighttalk/board/showthread.php?s=&postid=338120#post338120 ) I'm having a problem with a settled Mobil account and Calvary Investments. I'll summerize it here so you don't have to read the other thread...

I had a Mobil credit card which I defaulted on about 4 years ago. They offered me a settlement which I took. In retrospect this was probably not the best plan, but it's done. This is showing up on my credit report as a settlement with the note "Account legally paid in full for less than full balance.", so clearly it's been taken care of. The problem is that Calvary Investments thinks that it owns this debt and they're also listed on my credit report as an open collection.

Now, I already disputed Calvary Investments with the CRA's (which after reading some other recent threads here was probably not the best first step, but again, what's done is done) and I was going to just wait and see what happens, but now I'm thinking that it might be best to try to attack everything at once. I'm planning to send a nasty verification letter to calvary, but my question is what should I do about Mobil?

I see two options for this. The first is send a nice goodwill type letter saying something like "gee... I'm having a problem with a collection agency on an account that was previously settled. Could you please provide information on this settlement to me so I can provide it to them?"

Or I could go more for something like "We settled this account 4 years ago, but your collection agent is still on my back about it. Please do something about this or I will sue you"

While I do like the second approach a bit better the big issue is that unfortunatly I no longer have anything to document the settlement (other than that line on my credit report that says the account was "legally paid in full for less than full balance", and I don't know if that would be legally binding)

What are your thoughts on this?

Thanks again,
Brett