Using DRAM that runs at higher speeds is fine. Enabling these higher speeds (via XMP) is a form of overclocking and this can affect your warranty. Still, people do it all the time. It is important to understand that the success of any attempt is dependent upon a number of entities, the processor, the DRAM components and the motherboard itself. The issue is the motherboard's ability to minimize the noise on the memory bus (es) and the processor and DRAM component's ability to handle this noise. If the noise levels reach certain thresholds, these components may not be able to distinguish data from noise. When problems do occur, detecting which component is the problematic one can be extremely difficult.
An important secondary factor to understand is that, as components age, they produce more and more noise. Thus, it is possible that a system that works just fine when it is new may not work so fine once some time passes. We are talking years here, not months or days, so this is mentioned only as an advisory. If this happens down the line, disable XMP and see if this gets the system working again.
Hope this helps,
...S
