Try your stick with the 5:4 ratio and see what it does. Your 2.4's max is probably more than the 260 you're currently running. It will probably max in the 270-280 range (yes there are those that do better and there are those that can't break 250) which means that at 200 (and 5:4) you can run 250, to hit 263, 269, 275, 281, 288, 294 or 300, you need your ram at 210, 215, 220, 225, 230, 235 and 240, respectively. I would doubt that your current stick will do the upper end of that range but you won't know without trying it. If it does, then the cheapest way, obviously, is to get a second stick.
If it won't, then...
With ram prices so low now you can pick up a gig of something for under $100 that will run 220 or so and maybe even up to 240 or you could consider spending a little more and finding some Hynix, either bt-d43 or d-5. This will run you in the $120-$180 range, give or take, but you will be able to max your proc at 1:1 most likely, or for sure with the 5:4 ratio.
You should be able to notice a nice improvement running 270 at 1:1 (or even with the 5:4 ratio) in dual-channel vs 260 with a 3:2 ratio in single channel, as the increased bandwidth from the dual-channel alone would boost your performance.
Hope that helped more than confused.