I've recently gotten into the scene and already knew graphics card overclocking was going to make a big difference on performance. I initially started off with MSI Afterburner on my Asus 5870. I read around the various forums and kept spotting people underclocking memory. Unfortunately, MSI:A could only underclock to 600MHz at the lowest. Fortunately for me, I found AMD GPU Clock Tool (Call it AGCT).
Now I could really squeeze out that performance, but mind you I was still solo mining and this is on my own PC I regularly use for gaming. I didn't mind the need to manually switch back to lower, game stable clocks, but what I did mind is there is a low-power 3D setting in addition to 3D and full power 3D that was used solely for youtube videos. Playing -any- youtube video would cause an instant crash of my entire PC after using AGCT, even after restoring to default clock settings. Obviously something wasn't being reset. I banged my head against the desk several times mistakenly opening a youtube video and getting an unrecoverable freeze 1 second afterward for about a week. Then I got my Sapphire 5830 extremes.
It didn't take me long to realize it was impossible to alter voltage on these with either of the previous tools, until I saw "Trixx" being thrown out. I tried it out, lo and behind, voltage changing! This was well after the update to allow multi-gpu OC/UC/voltage. It doesn't provide GPU activity but I can live with it. I used the last card I had in my extra PCI-E slot in my main PC and opened up trixx, saw both graphics cards. I thought "Eh, beats using 2 tools" since I had to use afterburner for voltage and AGCT for clocks. Trixx successfully did voltage, clocks (including super low memory UC), and fan on my 5870, and the real kicker is it seems to preserve clocks for low power 3D, which means no more crashes!
TL;DR: If you are using AMD GPU Clock Tool to adjust clocks on a PC you use frequently for ordinary stuff (games, browsing, etc), I suggest switching to Trixx. It'll preserve the middle-level clock settings that can cause unrecoverable freezes if messed with by AGCT while allowing very low memory underclocks.