I don't like mismatched pairs either per say, but if you know what you're doing, this really isn't so bad, as long as the voltages needed is close.
I ran my 2x2 GB Mushkin & G.Skill just fine mixed...you just have to make sure you choose a good balance of vdimm & know what timings to watch.
(And yes, i OCed it quite a bit too )
Obviously, if you dislike what could be a bit more of a challenge, matching is a better idea.
Of course, if you are buying the second pair a lot later than the first, truth is they aren't likely matching anyway, as often the ICs get changed anyway over time...
I like the idea of getting a 2x2 GB kit, but not to replace the 2x1 GB kit.
Get 2x2 GB to add to your 2x1 GB for 6 GB
If you don't have a 64-bit OS...well, i'd say get one, but obviously that's not something everyone will want to do