I believe that the two newer games that are out that support dual core and Hyperthreading are Quake 4 and Call of Duty 2. Intel worked with both game developers to encourage the transition to multithreading.
The benchmarks for Quake 4 with the X2 3800+ vs. the A64 3800+ are below. The A64 4000+ that you're looking at will be slightly faster than the A 64 3800+ because of the extra cache, but this will give you an idea.
Call of Duty 2
City of Villains
World of Warcraft
Age of Empires III
Black & White 2
Peter Jackson’s King Kong
Battlefield 2: Special Forces
Tom Clancy’s Ghost Recon Advanced Warfighter
Tom Clancy’s Rainbow Six 3
Tony Hawk’s American Wasteland
Vangauard Saga of Heros
and believe quite a bunch more a supported or fixing to get supported...
Not to mention ATI's graphic drivers are multi-threaded giving an additional boost to dual core.