Actually Bane, you can get a Duron 650 for less than $50 US. Currently though, the price/performance ratio is IMO held by the Thunderbird 800. The duron 850 (which just came out this week) is the same price.
The Duron 600 will spank even a P3 800 for AH. The FPU is just that much better. (I've tested it, and I know it's true.)
My current primary system is a Thunderbird 700, with V770 Ultra (diamond 4.02), 128 mb pc 133 cas 2, Asus a7v (bios 1.004d). In forward view in fighters I usually get around 70 fps. The fps doesn't drop below 60, even in the biggest furballs. (It only goes to around 20 if I fly through smoke at a completely flattened base.) I did have this overclocked to 900, but there wasn't a massive increase in FPS, the V770 Ultra just couldn't go any faster.
I built a system for a relative with a Duron 700, 128 Mb pc 100 cas 3 (cheap stuff), but I used a Asus 7100 card (might have been 7700), which is a 32 MB Gf2Mx card. Using the 5.33b drivers that came with it AH ran at 85 fps (monitor refresh rate) almost continuously. This was in 1.04 patch 4 when I tested this system. In really heavy smoke it too dropped to around 25 fps.
I've also tested AH on P3 systems, none have come close to the Athlons for AH. Otherwise they are pretty close.
(I wonder if I could get farther off-topic.

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bloom25
THUNDERBIRDS