I made a Google Spreadsheet that will import the latest Mt. Gox trade value and your Mt. Gox Balance at the click of a button. then it calculates the two to show you their current market value. I has been very fun to watch the numbers grow recently. I feel like a gerbal in an experiment because I click the 'update mtgox Last Sell Price' so often.
Since it is a spreadsheet you can add your own numbers and calculations
I made a version as a public Template for everyone to use. if you go to Google Docs, File -> New -> From template...
Then search for 'yourBitcoins'
feedback is welcome
and donations too at 1ENTx3dEUE1KPhWYiAceEEYjnT7GWXeAjp