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It's hard to say exactly, but does sound like a compatibility issue. Some machines are very "fussy" when it comes to Ram. I would suggest you physically look at both sticks and compare. Are they both "single sided" or "double sided" (memory chips), or one of each? Have you tried the new stick on it's own? You say that with one stick, it does run @ 400, so I'll assume you've tried the new stick that way. How is it you are determining the Ram speed? Another note to remember, is that along with being fussy, some machines just don't work well with certain brand chips. You can physically have two sticks of the same brand and speed, but they can also have two completely different chips from one to the other. Some manufactures offer a "memory chart" that will tell you if a particular stick they mfg. is suited for your machine.
One thing that puzzles me though, are 100% sure you have a CPU that is ..
1- a "P4"?
2- a "P4" that has a front side bus of 800mhz?
Dell doesn't appear to have offered a 3000 with a P4 2.4, but rather a Celeron 2.4, key word "appears".
You can not simply go by the P4 decal on the outside of the box, as Dell is known for selling a computer specifically with a Celeron, yet still labeling the box with a P4 sticker.
Bottom line is, that unless you have a CPU with a 800 front side bus, your machine will not run at a Ram speed of 400 MHZ, but rather at 333 MHZ.
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