Currently only dual channel RDRAM can feed a Pentium 4's bandwidth needs. Unlike DDR, purchasing RDRAM is a little more tedious. Users of RDRAM must buy them in matching pairs in order to use them properly, this because of their 16 bit serial nature. Rambus has introduced new 32-bit RDRAM which works in parallel and which will shorty free it from the old constraints. The new RDRAM RIMM 4200's no can be used in single sets, but at the moment they are very not widely available.