Thanks for the replies, but I am trying to spend the absolute minimum to upgrade his existing system, just to play this game he is addicted to. I will buy him a better system in a year or two, so this is a temporary fix for a specific problem, just to get up to the minimum specs to play GuildWars2 (which is not a very demanding game).
I plan on getting him a RadeonHD 6850, or 6870 video card for $140 to $160 and want to keep the CPU cost as low as possible, so $85 for an Athlon 64 FX57 @2.8GHz, or FX60 @2.6GHz will be less money than a new motherboard @$60 & new CPU @$120. That is an extra $95 that I don't want to spend, unless it really doesn't make any sense at all to try to use the Athlon 64 FX57 @2.8GHz to try to run GuildWars2. If it can't do it, or if the frame rate is going to be too slow, even with the new RadeonHD video card, then I will have to spend the extra money and get the Intel Core i3 CPU and new motherboard. The new motherboard is going to require RAM as well and I don't think the price I quoted for the Intel Core i3 includes the heat sink and cooler fan, so those are other costs that need to be added as well.
Buying a used complete computer might be a cheaper solution, but I don't want to spend $400, or even $364, if possible.