Good question, personally i'd stick with triple sets...ie 2x2x2, or 1x1x1, etc.
Check your manual for what slots to arrange them in, but yeah, i'd definitely do that.
People get far too excited over having matching sizes/speeds/numbers when it comes to RAM.
If it's stable, 7 GB > 4 GB.
Any performance "drop" is going to be impossible to discern in anything but synthetic benchmarks.