I have a P4c 2.8 running stably at 3.26Ghz (233Mhz system clock) on an MSI 865PE Neo2 MB. With the case open, it seemingly runs stable at 3.5GHz (250Mhz), but when I bundle everything up, it ends up crashing every now and then at that speed. I havn't compeltely ruled out the RAM or the MB, but considering the memory is DDR500, I do believe it's the CPU (or it's stock heatsink/fan). Strangly enough, increasing the CPU voltage doesn't seem to help any; in fact, my CPU seems to completely reject any change to the voltage!
I made a major mistake 2 years ago when I built this system. While i'm fairly satisfied with it's performance, I stupidly bought 4x256Mb of ram for a total of 1GB. At the time, I was able to get each DDR500 module for $45 from Newegg. Now, however, I'm finding myself wanting 2GB of memory. So, of course, in order to upgrade now, i'd have to buy an entire 2GB of memory, which would cost roughly $180-$200. Couple that with a $200 video card, and i'm looking at a pretty expensive AGP system upgrade, which nearly negates the whole cheapness argument against upgrading to PCI-E...