Here is one instance where, at 2560x1600 at least, more RAM definitely DOES make a difference. Below is a chart detailing the engine settings used.
Source Engine Settings
Clock speeds alone couldn't help the 320MB GTS at 2560x1600 as the below graph shows. However, 1920x1200 was a different story with the extra clock speeds doing wonders for the 320MB GTS.
Counter-Strike: Source Stress Test
To test using the most demanding visuals Source has to offer, Half-Life 2: Lost Coast was used. The image below is from my favorite spot on this level, overlooking the entire bay. The screen was captured at 2560x1600 with all in-game settings at maximum and 4x antialiasing applied.
Half-Life 2: Lost Coast (in game)
Click Image to Enlarge - 2560x1600 (504KB)
The results are shown below in a point graph illustrating the gameplay in frames per second thanks to FRAPS. The area used to test was inside the chapel housing the cannon shortly after breaking said cannon. An onslaught of Combine in addition to lots of outside lighting from all over the room offers a beautiful and demanding area of the game. The below graphs show results at both 2560x1600 and 1920x1200 and the charts show a frame-by-frame analysis of the benchmarks.
Half-Life 2: Lost Coast Benchmarks
The charts below chronicle the second-by-second number of frames for each run-through of Lost Coast.
Half-Life 2: Lost Coast Benchmarks
Once again, the 320MB suffered at 2560x1600 in the Source Engine. And again, at 1920x1200, those extra MHz allowed the 320MB GTS to almost match the 640MB GTS. The Source Engine is definitely an engine that relies on very large textures so in this case, more memory is going to offer better performance at the highest resolutions.
