"How much of an improvement will I see by upgrading to 1Gb memory?"
The Setup:

I used 2x256mb matched set of Kingston HyperX and a 2x512mb matched set of OCZ's Platinum PC-4800.
Both dual channel sets of memory were run at 220mhz with 7-2-3-2 timings on 2.7v.
CPU: AMD Athlon64 3200 Venice core at 220HTT for a final CPU speed of 2200mhz (10x220) running on default 1.4v.
Motherboard: Chaintech VNF4 Ultra with V5.0 BIOS
Graphics: eVGA 6600GT 128mb PCI-Express at default speeds
PSU: Fortron "Blue Storm" 500w
The Test:
I set the graphics quality of nVidia's driver to "default" values which were determined by the application (which ended up being AA and 4xAF). All tests were run at 1024x768 resolution. Benchmarks included 3DMark2001, 3DMark2003, and Aquamark 3.
The Results:
3DMark2001 Overall Score:

3DMark2003 Overall Score:

3DMark2003 CPU Score:

Aquamark 3 GFX Score:

Aquamark 3 CPU Score:

Aquamark 3 Overall Score:

Aquamark 3 Average FPS:

My Thoughts:
Well, I was expecting a little more, but the results still show what I was hoping to see: and improvement. What suprised me the most was how much the CPU score increased in all benchmarks! But I was also suprised to see that the GFX score in Aquamark 3 actually went down a little.

Although I didn't test for it, I imagine I would probably see a slightly larger gap by running these same benchmarks with higher image quality applied (2xAA and 6xAA for example).
Needless to say, I think I'll be sticking with my new 2x512mb set of OCZ PC-4800


Let the overclocking begin!!!!!