There is definite diference between ValueRam and HyperX (kingston's other RAM).

Effectively, ValueRam lacks a level of ECC (error correction) that is required in high-performance environments (as used by servers, gamers, and programmers). It also lacks an extra thermal displacement system (cooling uint) that is required when RAM is being used heavily.

In a nutshell, what this means is that a "normal" user would probably never notice the difference as they ran thier day to day appications. If however, the user started running more advanced applications, they may start to notice thier computer crashing for no reason, etc... The best example of this is the latest version of Photoshop CS... Run it on valueram in a laptop, and the RAM swapping will cause the notebook to overheat and shut down. Run it on HyperX in the same laptop, you won't have this problem.

Finally, while all ram is spec'd at a certain speed (ie, PC3200) Value Ram is tested to work at the lower end of the speed spectrum, where as HyperX is tested to wok at the upper end.

As far as your other question goes, the general rule is never, ever mix ram types... if you do, the best result is that they will run together at the same speed as the slowest ram in the system... Worst result, they overload, cook themselves, and your motherboard. If you ever need to get new RAM for a system, try and make it as identical as possible to the original (even down to brand, though this is less important), or replace the original as well.

Take care.