bitcoin calculations chart

Hi

I am trying to put together an excel chart allowing me to calculate the time it will take to generate bitcoins (lets say with btcguild) based on the mhash/s created from my individual graphics cards and the current difficulty level (i know this would change in time so must be a variable in my excel spreadsheet)

Are there any smart people who have already done this?

eg headers may be

card eg 6990
mhash/s eg 800
diffictult level ?
bitcoins paid by btcguild per m/has - not sure on this one?

then we could add maybe cost of electricty and work out profit per day?

then add the cost of the card to work out when the card has paid for its self (we would need current value of bitcoin on mtgox)

I know that there a simiar online calculators, but Id like a spread sheet to monitor all my cards

then I will be able to see when per card it is no longer worth mining them and sell them

and as new tech comes out eg fpga and asic boards I can add these to the chart too.

Anyone good at maths that could guide / work with me on making this?

Thanks