It's 800 Mhz memory that's guaranteed to overclock to 1000 Mhz or 1066 Mhz (with increased timings and voltage). There's no JEDEC specification for it, therefore you'd have to set timings and voltage manually. The performance gains could be anywhere between 0 and 1.5% (depending on applications). You could also overclock DDR2 800 Mhz memory and probably achieve the same results.
The risk of running into issues always is lower when using memory that's known to work well with your motherboard.