G.skill F1-4000usu2-2gbhz Pc4000
 

The G.SKILL F1-4000USU2-2GBHZ unite overclocking and 2GB. They almost overclock like the best 1 GB Dual-Channel kits, but offer double the amount of capacity. We reached 275 MHz (DDR550) at low 2.6V. If you want the smoothness of 2 GB and also overclocking, they can be yours for about $245, which is an excellent price.

Link:
http://www.techpowerup.com/reviews/GSkill/4000-2GBHZ