Grand Total to
Estimated Total is easily accomplished by editing the
app/locale/ language file. For US, you would edit
app/locale/en_US/Mage_Sales.csv. On or near line 265 you'll see the following:
"Grand Total","Grand Total"
Change it to:
"Grand Total","Estimated Total"
The change is immediate.
There are several ways to add a small information line like the one you are describing. One involves creating an extension to add a new row in the Totals area but since you just need a single line of text that doesn't modify any values you could do the following instead:
app/design/frontend/base/default/template/checkout/cart/totals.phtml to your theme. (i.e.
Around line 45 you'll see this:
<?php echo $this->renderTotals(); ?>
Above this line you can place your message:
<div class="your-custom-style">shipping will be calculated at checkout</div>
The result will look like this:
I put the class "your-custom-style" as an example showing that you can style the text any way you want.
This is assuming you are using the rwd theme. The same should work with any custom theme. The important thing is that the file gets into the correct location.