Thats not completely true, running PC2700 with an 939 doesn't give a large performance hit. I ran my 3500+ on Hyper X PC2700 up til recently and thru all benchmarks i've run thus, its at MOST a 15% increase. Also, with larger modules 1GB and + some mobos have issues running memory at 400MHz. Eg. my Epox nforce 3 Ultra mobo would ONLY run @ 333 MHz if using 1GB modules I believe. There's prolly a bios fix since, but I am just siting some possibilities.