Have you considered there may be other factors causing your lower performance than your configuration?

Strange things that have caused my systems to "under perform"

#1 External FPS thieves:
SOUNDCARDS - especially onboard sound with hardware acceleration. This is a quick and easy test. Disable the onboard sound card in BIOS. Re run your tests. Even if it's disabled in WoW, hardware acceleration can cause all kinds of performance issues behind the scenes, even crashes. The Asus P5K gave me all kinds of troubles with Vista64 until a software driver update.

USB Devices - "a good USB device is an uplugged USB device". I spent weeks in Vanguard tracking down troubles, ended up being conflict with my G15 keyboard and a headset. Try testing with a straight USB mouse/keyboard and no other USB devices plugged up.

RESIDENT PROGRAMS - Antivirus, firewalls, crap that loads when windows launches. Try disabling all those, including the autorun's and test again. I had issues with Free AVS antivirus software, and a media center extender for the 360 3rd party.

#2 System temperatures:
One thing I have been learning about 3870's and in all cases multiple 3870's is they are hot. Too hot. Use a software based temperature monitor to keep track of both GPU and CPU temps. SpeedFan was a savior for me, when I had a 3870 that during peak times would puff out 150 degree heat into the case. Most CPU's like your quadcore, and some motherboards, will put in Throttling to protect the CPU from overheating. This can look like stuttering or complete slow downs for no reason.
Open up your case and place a floor fan on it while running, test it and see if it helps.

Bad hardware Settings
Having weird refresh rates or odd driver parameters. I had one of my computers hooked up to a rear projection HDTV during game downtimes to watch movies. Having VSYNC on and an odd 59Hz refreshrate to be compatable with this TV was wrecking havoc when I toggled back to my LCD monitor and playing games. Removing all those settings (and a program called powerstrip) made these issue go away. Set and double check all your settings regarding refresh rate, let the game API set the defaults for you, don't override anything in the control panel. Test again.

Bad hardware
A single bad stick of RAM is hard to identify. Silly things like a crimped or damaged IDE hard drive cable. A couple bad sectors on a hard drive, can all lower over all game performance and not really be noticed. Most hardware has to attempt to access memory or a file location several times before failing and reporting back to windows there is an error. These things can be a terrible pain to track down. Removing them one at a time, using "known" good equipment to compare performance to can help uncover them. Most the time I only discover it when I upgrade along the way and realize that the part I just traded out was bad.

Bad Karma/The Charlie Brown Theory
Sometimes bad system performance happens to bad people. Often I have bad system performance and will spend some time doing positive things, like taking out the trash, or walking the dog, reading to the blind and try it again. I know this isn't very scientific but alas when all else fails and the choice is to stop playing your favorite games or drop 4 grand on a new computer in desperation, a little good Karma can go a long way.

And you techno nerd out there, please do tear apart my suggestions, these are just some common things I've seen on my computers over the last couple years, that made my head hurt and my games to lag.