Many rate the 6600 DDR2 256 as a "Best Buy"... and it is a verygood deal. The DDR2 increases performance markedly over earlier DDR models. But the FX6600GT 128MB GDDR3 is great, for just a little more. 256MB might be nice for games in the future, but even BF2 can run at top settings with this card. The only thing you 'give up' vs a 256MB GDDR3 is about 10 or 15% higher framerates in some games in the 1600x1200 league. The 128MB did require a very few games, like Black & White 2, to deactivate AA from 1024x768 and higher due to the 128 MB. The difference in lower resolutions is not significant, and 128 is not a show-stopper (e.g., you do not really need the extra 256, in the case of either DDR2 or GDDR3). Both will do RTW, but the GDDR3 will do it better.

So given the choice, get the GDDR3! No question.

GeForce FX6600GT 128MB GDDR3 16x PCI Express Video Card
General Features:
Nvidia GeForce 6600GT GPU
16x PCI Express interface
128 MB GDDR3 memory
128-bit memory interface
0.11 micron
400 MHz RAMDAC
CineFX 3.0 Shading Architecture
64-Bit Texture Filtering and Blending
Intellisample 3.0 Technology
UltraShadow II Technology
Advanced Video and Display Functionality
Advanced Display Functionality
Advanced Engineering
Nvidia Digital Vibrance Control (DVC) 3.0
Complete DirectX support
DX9.0 Shader Model 3.0
Full OpenGL support, including OpenGL 1.5
Integrated HDTV encoder
Up to 3x Geometry performance

But for comparison, here is a GeForce 6600 PCI Express with DDR2:
DirectX support: DirectX 9.0c Shader Model SM 3.0
Vertex shaders: 3
Pixel pipelines: 8
RAMDAC: 400MHz
Manufacturing process: 110 nanometer
Transistors: 146 million
Core clock speed: 400 MHz

Both cards utterly destroy the typical competition, the X1300Pro.
Both cards are utterly destroyed by the top of the line nVidias .

For quick reference, here are some comparisons discussed in Toms Hardware Guide, from http://www.tomshardware.com/2005/12/02/vga_charts_viii/index.html.

Note: Look at the RELATIVE differences... these were tested on a top-flite rig (FX57 + 2GB UltraLowCAS RAM).

Neilus, if it's possible, get the non-Extreme version of the 6600GT. I feel that it's not worth paying the extra for an Extreme version, which is basically a pre-overclocked normal 6600GT. Don't worry, it's also GDDR3.