Lets get this CPU speed in games into perspective eh? (I thought the folks you were building this for didn't do games anyway? But, that's by the by...)
Take Far Cry at 1280 x 1024. According to Toms Hardware a Pentium D820 and an X2 4200 achieve the following (They don't specify the D805, so I'm using the 820 to represent a mildly overclocked 805):-
Pentium D820 = 132.5 Frames Per Second
AMD X2 4200 = 179.4 Frames Per Second
So, a difference of 46.9 Frames Per Second. Great.
What does that mean though? Not a lot I'd say, seeing as anything over the range of 35 to 80FPS is almost irrelevant to most human eyes, and your average TFT can only manage to display 75FPS, you can see why I think we worry too much! (Enabling Vsync can change things, but that's another can of worms!)
The conclusion I'd draw is both are fine for games, so don't worry about it. Your graphics card is more important.
The Athlon 64 FX57 scores 213 or so in the same test by the way, 33.6 more FPS than the X2 4200, but nobody goes on about how rubbish an Athlon 64 FX57 is in comparison!