you should not have any problem with stability or performance at all, unless you are massively overclocking.
it is always best to get entire matched sets, and with memory prices being as low as they are now it probably wouldn't cost much more to get a pair of 4GB DIMMS, but that's between you and your budget.
Don't forget that if you go above 4GB of RAM you will have to switch to a 64-bit version of windows, or else your system will only recognize 4GB.
Also with your processor I would suggest 1600MHz, as it seems to be the "sweet spot" for Sandy Bridge processors. So, good job with the kit you have
getting anything faster won't really yield you and performance differences.