I know there are many tools to check an ecdsa signature of a bitcoin transaction and I understand the ecdsa algorithm. It takes three inputs, a hash of the transaction, the digital signature and the public key of the sender.

In a raw transaction such as this one: http://blockexplorer.com/rawtx/9b4912711de5e258a74a529227f43647bc3b2304212fb508c5f8e910d5986270

I know the signature is the first line of the Scriptsig and the public key is the second and I assumed that the hash of the transaction is the first line on the page (also in the URL) but they don't validade and I've tried a handful of transactions from blockexplorer.

My question is, what do I take as the first variable to validate a transaction in that form?