Like Supersaki said, the 'About Phone' section isn't always accurate. SetCPU is probably the best way to see what speed your CPU is running at. Also keep in mind that not all kernels work with all ROMs and Android versions. The hardware in some funs will run 1.25GHz Ultra Low Voltage kernels no problem, some just won't do it. Get your phone running stable with a stock ROM/Kernel and make a nandroid backup (through SPRecovery or ROM Manager) then start trying out different kernels to see what works for your particular set up. Usually I start with the max speed I would want running the lowest voltages (typically a 1.1GHz ULV). If my phone runs stable that's what I'll stick with unless it eats through my battery too fast. If its not stable, start backing off the speed or switch to a LV or Standard Voltage.
As far as ROM Manager and ClockworkMod go, don't give up on them. They can be a little finicky at times, and have had several issues lately, but its a great platform and a great resource. There is a known issue with ClockworkMod Recovery getting broken when you first flash it. The easiest solution is just to go into ROM Manager and click 'Flash Alternate Recovery' down near the bottom. This will switch it to SPRecovery, then press 'Flash Clockwork Recover' near the top. Should install correctly from there, but there is still and issue with the latest version of Clockwork not being able to install some ROMs (such as Sapphire 0.8.4).
Being able to find ROMs or Kernels there is dependent on the developer getting it set up. Last I checked most of the kernels I use weren't on ROM Manager, I think only P3 and Beckit are on there. Those two certainly make some good kernels (and they are both largely to thank for kernel development as we know it), but I usually have better success with ChevyNo1 and jdlfg (I think that's his name, he's on Cyanogenmod forums) kernels. My phone just likes them better. You can also find some kernels written specifically for Sapphire here: