Well, the 2x1gb RAM is more expensive cause it leaves you room for expansion. Consider a 4-RAM slot mobo. If u fill all slots with 512MB RAM sticks, it will be 2GB but if u want to upgrade to 4GB, for instance, in the future u will need to get rid of all four 512 sticks. But with 2 sticks of 1GB u will just need to get two more 1 GB sticks.
Also, for many mobos they only have 2 RAM slot, so obviously they will have to buy 2 sticks of 1GB Ram rather than 2x512 RAM.
DFI Lan party UT NF4 Ultra
AMD Opteron 146 2.0 Mhz @ 2.8 GHz
OCZ Gold Edition 2x512 MB DDR 500 timing 2.5-3-3-8 @ DDR 560
XFX GeForce 7600 GT 256 Mb GDDR3
Antec Lanboy Mini Tower Case
Antec True Control 550W ATX2.0 PSU
Maxtor 300 GB 7200 RMP SATA 16Mb Cache
17\'\' TFT Screen 12 ms