After our Doom 3 - CPU Battlegrounds article, we already knew what to expect from CPU performance under Half Life 2. At this point, Intel's Pentium 4 architecture does take a backseat to AMD's Athlon 64 when it comes to gaming. With performance advantages of around 15% at the same price point, the Athlon 64 does offer a noticeable increase in performance over the Pentium 4 in Half Life 2. The extensive physics engine in Half Life 2, especially as seen in Half Life 2 Deathmatch, is very CPU dependent and thus we see a real world performance advantage to the Athlon 64 over Intel's Pentium 4.
Not only is the game very CPU intensive, but because of its dependency on a fast CPU, Half Life 2 also appears to be quite dependent on high available memory bandwidth and low latency memory at the same time. Half Life 2 is actually the first game where we've seen this degree of dependency, which does make for some interesting predictions for the CPU/platform requirements of the next generation of games. Despite what we have seen in recent years, it does look like the next generation of games that employ more sophisticated artificial intelligence and physics will be quite CPU and platform dependent, just as they are GPU limited. The balance will obviously vary from scene to scene in the games, just as we've seen with Half Life 2, but the limitations will be there.
In terms of the right speed CPU to pair up with your GPU, if you have a high-end GPU (X800 or 6800GT class) then the faster you go the better off you are. Mid-range GPU owners will find that anything the speed of an Athlon 64 3000+ (Socket-939) will offer the best bang for your buck, with diminishing returns after that. If you happen to have an older Radeon 9600/9700/9800 based card, then even an Athlon 64 2800+ will be overkill for your GPU. If you are stuck with one of those older but still well-performing GPUs, don't bother upgrading your CPU unless it's something slower than a 2.4GHz Pentium 4 - you'd be much better served by waiting and upgrading to dual core later on.
The impact of the CPU on gaming performance is in a transitional stage right now. As more games use Half Life 2 style physics we will see similar impacts with regards to CPU performance, but at this point there's a great deal of work being done on multithreading game engines for the next generation of games. So while the games coming in the immediate future may behave similarly to Doom 3 and Half Life 2, it's the games that follow that will truly be interesting.
Check back in the coming weeks for more information on multithreading, dual core and the future of CPU performance in game engines among other areas...