How do I get started mining bitcoin with a GPU?
I just did this today...

I found a mining pool (I picked Eligius) from here https://en.bitcoin.it/wiki/Comparison_of_mining_pools

The pools website had further instructions. In my case, I installed a mining client "bfgminer" and followed the instructions to add my ID. It sounds like for a windows OS, GUIMiner might be a good start too.

gui miner thread https://bitcointalk.org/?topic=3878.0

The key thing to realize is that there is a difference being a BitCoin "client", which is what you setup above, and a BitCoin "miner" which requires the steps I in this response.