Bought a new CPU AMD 2400+XP replacing old 1.4 gig Athalon and saw a 15 - 20 fps increase in second beta ( tried an tested on both CPUs before ya all start
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So my advice is if ya want AH frame rates on AH2 and your video card is OK consider a relatively cheap CPU upgrade to max out your mobo.
Obviously not everyone is going to have a cheap upgrade option but check it out you may be very pleasantly surprised how cheap it was and how much difference it makes.
Adjuster
A complete idiot at upgrades but even I could do this with 2 screwdrivers and 10 minutes to spare.