AnandTech has reviewed a bunch of DDR memory kits with a 2GB capacity from Corsair, Crucial, G. Skill, Kingston, Mushkin and Team.
There are many reasons to choose a 2GB kit over a 1GB kit or 4 512MB DIMMs. Two 1GB DIMMs on the AMD Athlon 64 can still run at 1T Command rate, instead of the 2T required by the 4x512MB DIMMs needed for 2GB with 512MB DIMMs - a definite advantage for the 1GB DIMMs. On the other hand, until recently, the available 1GB DIMMs were generally much slower than the fast 2-2-2 DIMMs that were commonly available in 512MB DIMMs. We normally saw 3-3-3 or 3-4-4 or slower timings for 1GB DIMMs. These poorer timings for 1GB DIMMs took away most of the advantage for the 1GB 1T Command Rate.