Users of PowerPoint 2010, as well as users of PowerPoint on OSX, are unable to join in on the 300 dpi goodness. For instance, in PowerPoint 2010, there is an option to scale images to 220 dpi, but that's not the same as 300 dpi, is it? In OSX, this functionality was included in PowerPoint 2008, but was removed altogether in 2011!
Here is a quote from someone working at Microsoft Support,
"Unfortunately, the "Dots per inch" option is no longer available in Office for Mac. If this is a feature you'd like to see in future versions of Office for Mac, be sure to send your feedback by clicking..." blah.
I propose a solution similar to what I have recommended for creating high quality conference posters. It requires that you have a PDF printer installed (below I use the example of Acrobat, but the concept applies equally well to other PDF printers).
Step 1: Create your graph in Excel. I recommend leaving the axes unlabeled for the time being to avoid distorting the text when resizing the figure.
Step 2: Print the image. With the image selected, select File | Print and then set the Printer to Adobe PDF (or whatever PDf printer you have) and select Printer Properties.
Step 3: Edit Image Settings. Edit the Images setting so that the PDF creation process will preserve the image in high quality without downsampling or compressing it. Click OK and optionally save your settings for next time. Print your figure to PDF.
Step 4: Extract TIFF image from PDF. Open your newly created PDF in your viewer of choice, and savethe page as a TIFF, setting the resolution to 300 dpi.
Step 5. Add finishing touches to TIFF in image editor. Open the TIFF file in your image editor of choice (I use Paint.NET). Add labels to the axes and legend (tip: for the x -axis, it is easy to keep the labels aligned in vertical space by using the text tool only once and separating the labels using the space key to position the labels along the x-axis). Crop the image and save. The 300 dpi resolution will be preserved in your newly cropped TIF.