Well, some RAM needs a certain voltage to work under. The default JEDEC standard for DDR is 2.5-2.6, however, not all memory chips are designed or are able to properly operate under that voltage. Both my Patriot PC3200+XBL and Crucial BallistiX need 2.8V to really work right. It's not a design flaw, it's just a certain requirement of this RAM. Some RAM will work at lower voltages, but if you start to overclock, then you'll need to bump up the RAM. Windbond BH-5 and CH-5 RAM chips pretty much require 3V or higher to really 'clock. However, as I've said before, they also need cooling because the voltage going through the RAM adds extra heat needing to be dissipated.
Any RAM can be tweaked and overclocked, however some RAM will do this 'better' than others. Besides the Patriot +XBL and Crucial Ballistix I mentioned, there's Corsair XMS, OCZ's VX, Platinum, Gold, and XTC, Kingston HyperX and G.Skill HZ lines. These are produced for tweaking and overclocking. This isn't to say if you got some ValueRAM from Kingston or Corsair, you've bought crap. ValueRAM is made for systems that most people are going to use: surfing the internet, burning CD's, playing music, word processing...the basics. Tweaking isn't a big deal in these cases. Until last year, I didn't use any high-performance RAM, but my DFI NF4 Ultra-D happens to require high-performance RAM for best results.