There is very little difference between 512, 1024 and 2048 in 3DMark 05. I've copied in from an earlier post (at the bottom) some benches with 512, 1024, and 2048 in both PCMark 05 and 3DMark 05.
Overclocked
Asus A8n-sli Prem 3800+ X2 and sli xfx 6800GT's 2x1024 (current rig)
3DMark05: 10752
Jetway 3000+ and sli xfx 6800GT's 2x512 (old rig)
3DMark: 9416
Stock
Asus A8n-sli Prem 3800+ X2 and sli xfx 6800GT's 2x1024 (current rig)
3DMark 05: 8952
Jetway 3000+ and sli xfx 6800GT's 2x512 (old rig)
3DMark 05: 8243
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Copied from an earlier post...
The difference between Dual Channel, Single Channel and 512mb, 1024, and 2048mb's of ram in a system, while benching them against 3DMark 05 and PCMark 05.
Base Settings:
3800+ X2 (@ stock 2.0ghz)
Asus A8N-SLI Premium v1009
-OCZ Platnium 3200 2x1024(2 3 2 8 @ 200fsb)
-HyperX 3200 2x512(2 3 3 8 @ 200fsb)
WD 60GB
SBA2
SC HyperX 1 module
Pic 3DMark 05 – 8772
Pic PCMark 05 – 4561
SC HyperX 2 module
3DMark 05 - no boot
PCMark 05 - no boot
DC HyperX 2 module
Pic 3DMark 05 – 8912
Pic PCMark 05 – 4609
SC OCZ 1 module
Pic 3DMark 05 – 8770
Pic PCMark 05 – 4559
SC OCZ 2 module
3DMark 05 - no boot
PCMark 05 - no boot
DC OCZ 2 module
Pic 3DMark 05 – 8928
Pic PCMark 05 – 4606
Excel Spreadsheet