We do something very similar at my work, except we dont use index cards (in part because my handwriting is atrocious (damn that Palm graphitti!) and because I type much faster than I could ever possibly write).
When we have to estimate a task, we start out with a list of the functionality the client wants. Then we break it down into the tasks we need to take to accomplish that functionality. Then we begin estimating how long things will take.
We then have a few general rules we follow.
Once the estimate goes off to the client it is out of your hands. However, this does not mean that everything is written in stone. Proper handling of scope creep is critical. I used to be hesitant and uncomfortable when dealing with clients on scope creep. But I have realized over the years that you get more respect from a client, and get taken advantage of less if you are hard and fast about scope creep. If explained properly, your client will understand and agree, and you can negotiate. If a client is too pushy and want's you to do it for free, then they are clearly trying to take advantage of you, and you need to respond appropriately. Always remember, your clients are not your friends. That is not to say that a particular contact at a client might not become your friend (I have worked with one guy for 3 years now and we are good friends), but when dealing with matters of business, friendship and feelings need to be left at the conference room door.
Another thing I have found is very helpful, is to make sure that client deliverables are on the schedule and those dates are enforced. Many times my estimate has been derailed because they were late, however since compliance was not enforced, my schedule was never changed, and I ended up working nights and weekends.
I say client many times here, but within reason you can substitue "account guy" or "project manager" for client (taking into account the subtleties of office politics of course).