PrPro CS5 added the Mercury Playback Engine (MPE), and uses CUDA technology provided by some nVidia graphics cards. This Blog, by Todd Kopriva of Adobe, goes into a lot of detail, and will define MPE completely, plus list the certified nVidia graphics cards.
Now, a common question is, what does this do for me with PrElements. As of PrE 9.0, the answer is "nothing." However, none of us knows whether some of MPE will appear in a later PrE version. Only time will answer that.
As the question of graphics cards comes up often, one possible consideration, when making a choice, is - "will I ever move to PrPro?" If the answer is yes, or even maybe, then consideration might need to be made to allow for CUDA hardware acceleration. At this point in time, without MPE, or CUDA support in PrE (as of version 9.0), the potential for moving to PrPro would be the only one. However, as stated above, none of us knows if PrE will have MPE, and utilization of CUDA in PrE 10, or later.
For more detail on MPE and Effects, see this ARTICLE.
Hope that this helps explain what MPE and what CUDA are, and how they can affect the choice of a video card.