Mixing DDR brands
 

I've got 4GB of Corsair DDR2 , I was going to add another 4GB but can't get the same memory from where i'm buying from. They have G.Skill DDR2 , same speed and specs though for a good price. It shouldn't - but will mixing up these two in the motherboard cause problems?

It shouldn't cause any problems, but try and get memory with similar if not identical CAS timings 8-8-8-24 2N <- these numbers should be the same for both sets if possible as well as ECC or non ECC, also check voltages as they can differ. If you are not sure of the timings of your current dram use something like "sisoft sandra", this will allow you to check the timing's, voltage etc of your current memory.

If you cannot get identical timings, try to make sure the new stuff is faster than the old, with CAS latency, lower is better; this is because some memory controllers will default to the slower set of timings for both sets, while other dual controllers can run both sets at different timing. Adding slower memory could reduce the performance of the memory you already have installed.

Probably the worst that would happen is no dual channel which in reality doesnt make a huge difference anyways.

From memory benchmarks Ive seen speed make more difference than CAS timings and such

You're going to need to loosen the CAS timings anyway to get the 8Gb of RAM to work in 4 DIMM slots.
You wouldn't lose much by buying 2x4Gb sticks, sticking with dual-channel, and keeping to the tighter timings. Memory is very cheap at this time.

You want same speed, same latencies and same voltage. The first two are less important than voltage, because your motherboard usually sets them to the lowest spec'd pair anyway. You can set the sticks up in your BIOS to match eachother but if one higher binned set needs 2.0V, and you pair them with a set that needs 1.8V, some serious testing needs to be done with both sets individually before you run both. You need to know that all sticks will run ok with the lowest voltage. Or highest. You also need to know the recommended voltage range(max/min) of each set.

Say that you run the 2.0V sticks at 1.8V, then it's not 100% guaranteed that they will run ok. Performance loss is the result.