Excellent review.
It shows that despite Athlon XP being generally slower, it does have an advantage of using 3 (or perhaps 4) memory modules and thus having 3GB RAM at DDR400 speed. I used to have an nForce 2 board with 4 DIMMs.
Athlon 64 with AGP is about 15% faster at the same clock speed and cache, but there are 2.4Ghz and 1MB cache models that would push it about 8-9% further without being too expensive. 23-24% faster is noticeable in games. 2.4Ghz 512kb L2, 2.2Ghz 1MB L2 are the CPU models worth getting or one has to OC slower model. Besides having the 12V P4 CPU power connector, another advantage is quiet CPU cooler availability. You can replace the fan on stock s462 heatsink with Noctua but they have low airflow.
When it comes to the issue of Quake 4 memory swapping, I have not observed it with 2GB RAM as long as other apps are closed and GPU has sufficient memory. Probably at least 512MB is needed, GTX 260 has 896MB and it is definitely sufficient. Quake 4 and FEAR are not something one would play on AGP, even though details could be reduced to make it work.
Pentium III 900E,ECS P6BXT-A+,384MB,GeForce FX 5600, Voodoo 2,Yamaha SM718
Athlon 64 3400+,Gigabyte GA-K8NE,2GB,GeForce GTX 275,Audigy 2 ZS
Phenom II X4 955,Gigabyte GA-MA770-UD3,8GB,GeForce GTX 780
Vishera FX-8370,Asus 990FX,32GB,GeForce GTX 980 Ti