The Corsair XMS PC2-6400C4 comes to bridge a gap for many users with its low price while still providing very good overclocking capabilities. This product includes EPP profiles for 400MHz with 4-4-4-12-2T timings. So, if you have an EPP enabled motherboard, you can have the highest possible performance with lower timings.
According to our tests, the memory reached 563MHz with a voltage of 2.3V! That's 163MHz higher than what Corsair rates the product at. It's quite possible that with higher memory voltages, you can boost more MHz, but this is not suggested. With lower speeds, you can have 1T at 333MHz, but this only applies to Nvidia 680i chipset motherboards.
Ending our presentation, we are very satisfied with the Corsair PC2-6400C4 memory series. The product does work as advertised, and with the proper motherboard and memory voltage, can provide an additional 163Mhz for your overclocking needs. The retail price is really "low", at around €150~190.