As always, overclocking results should never be taken as gospel. Your mileage will always vary. Here's a shot of the utility that Matrox provides in order to overclock the card.
The one thing that might be confusing, particularly if you're used to NVIDIA cards, is that this utility sets the overclocking percentage rather than the absolute frequency(s) for both the core clock as well as the memory. As an example, consider setting the utility to the value 10. Since the chip is clocked at 220MHz and the memory at 550MHz, this would yield a core clock of 242MHz and a memory clock of 605MHz.
During my testing, using all stock components, I achieved a 5% overclock.
Let's see how this translates in real world performance.
Quake 3 - Overclocking Results
Average Frame Rate
The general consensus is that the Parhelia is basically clocked very close to the maximum stable frequency, so one shouldn't count on being able to overclock the board by a significant amount. Most people were somewhat surprised by the relatively low clock rate to begin with, though understandable given the complexity of the chip.