Thats a good question.
Yes it has the same pin number as the Inspiron. However, I believe that the FSB on the Inspiron only goes up to 533 so the 667 would be throttled down to the max FSB of the motherboard. Even if it could fully use the 667, it would still throttle down if you are combining the two sets of ram. Memory is set to run at the lowest frequency between chips. In other words if you use 2 stick of DDR2 800 and 2 sticks of DDR2 400, it would only run at 400mhz. Will it hurt your machine even if it doesn't work? No it shouldn't. One of two things will most likely happen. 1) The mother board does what I said before and drops the ram down to 533. or 2) The motherboard doesn't recognize the new ram at all and you simply remove the new stuff.
I just checked online on Dell's site. Your system can only support 2 sticks of Ram up to 533. In other words, unless you only have 512 (2X256) mbs of ram it would not be worth it to use the new ram.