Indeed with the Athlon 64 3800 the framerate improves by 2-3 FPS - empirical testing by playing the game. Probably with the FX-55 you would get ~50 FPS which makes for a pretty good experience (in FEAR it is highly important to have above 40 FPS otherwise the game feels heavy).

Benchmarks: one important criteria is to have some kind of automatic tests even for games so the results could easily be checked by everyone. I dropped all directx 7&8 benchmarks with the sole exception of 3dmark 2001.

Synthetic tests:

DirectX8: 3dmark 2001;
DirectX9: 3dmark 2003, 3dmark 2005, Aquamark3;
GUI test: Tom 2D;
CPU tests: Super pi, Cinebench 2003;
System tests: Pcmark 2002 & 2004 & 2005;

Games:
DirectX9: Farcry, Half life 2; Serious Sam 2, FEAR, Splinter's cell chaos theory;
OpengGl: Doom 3, Quake 4.

I must underline the fact that I have performed all synthetic tests at the default settings first and at maximum details in all subsequent runs. All games have been tested fully maxed out with the exception of Doom 3 and Quake 4 which are too demanding even for GTS 250.
Because I am testing CPU's I do not see the point in testing many resolutions. Most probably 1024*768 would be sufficient, maybe 1280*1024 if there are requests. I dropped 1600*1200 as it is very tough on the video card.

I did tried again Need for speed most wanted but it is impossible to get repeatable results - the framerate varies wildly between the runs even if i choose the same challenge/race so I dropped it from the testing suite.

Now on to testing...

Playing FEAR with both soft shadows and AA 4x is impractical as soft shadows get disabled after restarting the game, even though they show as enabled. This is evident from benchmark results. You don't want to enable it every time you start the game. Therefore I just enable AA 4x.

You want to test CPU in practical scenarios too - 1280x1024 at least. The question is how much newer CPU would help you in practice, not just in low resolution.