Best a Dell Dimension 8400 can get.
If you still have an old Dimension 8400 chugging along, you might as well upgrade to the single fastest CPU is can take. 3.8ghz on two logical cores doesn't even sound that bad. Sure the old architecture is outdated, but if you are still running an 8400, you know very well that it doesn't take the shiniest new parts, and the fastest, best performing chips to make a damn good, reliable computer. Show your clunky old Dell some love, and throw the Pentium 4 670 Processor in there.
Just, make sure to use some better thermal paste than the stuff they include, or else your CPU Fan will throw a fit. With good thermal paste (I recommend Arctic Mx-4) the CPU Fan will be a little louder, but the folks that say that it increased the fan's noise by a lot probably just used the low quality thermal paste that was included with the CPU. Just throw that stuff out.
Verified purchase: YesCondition: Pre-owned