There's a point where diminishing returns kick in when buying fast RAM. I'd say it kicks in for anything over 2133MHz. The extra performance you get after that point is so small that you won't notice it in real world use. That little difference will only show in benchmarks. Many people think that the higher the MHz, the faster the RAM or the lower the CAS, the snappier it gets. Most of the time it's not true. It's a little bit of both. When buying or comparing RAM, it's important to calculate their
efficiency.
Back to your TridentX 2400MHz issue. By now you've probably investigated on Google and found out that there are actually a big difference between the first kits that were released and the ones after that. If you got the double sided ones, you could overclock then as you wish. But if you have the single sided ones, then you will only be able to get a few extra MHz by moving the BCLK.
I was never able to overclock mines either. Whether pumping 1.75V on it or loosening the timings. The best I got was tightening the timings to CAS 9 @ 2400MHz but pumping a whopping 1.78V on them. Even as that, they weren't SuperPi 32M stable.