With the old ram your total ram will slow down to the lowest speed of the two and would cause a slight (hardly noticable) slowdown of your system. However, if total ram is more important to you than speed it's worth it. What do you do with your PC? Also, ram is cheap these days so why not just get some more fast ram?
Yeah true, I could just buy some of the same brand soon anyway. I use it a hell of a lot, so speed is probably more important than ram. I use it for digital design, affer effects (hence why i wanna make things quicker), gaming etc etc.. I guess I use the PC for everything. haha. Just having the new 8GB ram in, I can see a massive difference in speed compared to before, so I guess Im best to get some more new ram of the same brand so the speed is improved further, rather than just the ram speed?