If you're heavily overclocking, having 4 sticks can (and most likely will) affect your success. You also introduce more chances of having a bad stick, either initially or developing over time. That's nitpicking however, and the costs of replacing 2 sticks can't be overlooked.
There was an article posted not that long ago where someone benchmarked 4 sticks versus 2, and the 4 performed slightly better. That's the first I've seen of benchmarking such a scenario. I agree w/ the above poster though, that the difference is negligeable.
Running PC3200, my hunch is that you're not overclocking, so by all means, fill up those slots and save yourself the expense of needlessly replacing your existing dimms.