I love the idea behind Vista's window-preview thumbnails, which pop up when you mouse over any running program in the taskbar, yet I find them almost useless. Why? Because the thumbnails are just too small. Fortunately, I've discovered the awesome Vista Thumbnail Sizer, a free utility that lets you make thumbnails just about any size you want.
After extracting the Zip file, open the 32bits or 64bits folder (depending on which version of Vista you have), then run the AveThumbnailSizer application. Use the two sliders to set the maximum width and height for your thumbnails. I've got mine at around 550, but you should experiment until you find the settings you like best. The changes are instantaneous: After you move a slider, you can mouse over any program to see the newly resized thumbnail.
When you're done, click Start AveVistaThumbnailSizer with Windows so that your settings stay in effect after you reboot.
Note that Vista's Aero effects must be turned on for this program (and, indeed, Vista's window preview) to work.
Note: This program is donationware. It is free to try, but the author accepts and encourages donations towards further development. This link takes you to the vendor's site, where you can download the latest version of the software.