Where did you read that those Corsair sticks will hit 275MHz on loose timings?
Anywho, if you haven't changed your RAM divider then you simply start raising your HTT from 200 on up.
You will need to make sure that your CPU has enough Vcore for stability and that your RAM has enough Vdimm for it to be stable just like the CPU requires extra voltage.
Other than that you basically have the right idea. The further you go up, the more you'll have to loosen your timings. You gotta find that area where the timings are just tight enough with the higher fsb or in your case HTT, that you are infact getting more bandwidth through.
If your ram is 2-2-2-5 timings at DDR400, then it will probably need to be around 2.5-3-3-7 for 240FSB, sometimes it requires doing something like 2.5-3-3-10, that cycle time has little effect on bandwidth but a good deal of effect on stability.