will this work on my dell?

Maybe

probably not, this is performance ram and dell != performance.

#1 this is ddr2 RAM. Does your dell use DDR or DDR2? If the latter sure, if the former no. Unless you have a dell server which probably uses DDRx ECC and then no. You can only have all ECC or all non-ECC. If you do not know what any of this means, then put your dell back in the box and ship it back to dell. BTW, if you have a dell laptop then the answer is no regardless of what you have.

It'll work but you'd be wasting your money if it only supports PC2-4200 which, I believe, applies to most Dells. Even XPS's only go up to PC2-5300.

You can buy Dell's with 667MHz memory if you pay attention to what you buy. I have not seen anything faster than that. Most are 533MHz. Therefore, you are probably wasting your money unless you use them in something else down the road.

#1 It depends on what system you have. The XPS systems have higher capabilities than what normally people think. I have a XPS410 and it allows DDR2 PC2-6400. The only downside is that you will have to either sell the previous one installed to maximize the speed of your RAM or give it to someone else who needs it.
For those people who will ask me why I bought dell ... it's just because it is cheaper with the basics only in it and the software that comes with it. I tried to build one and my savings is not that much. I can upgrade the system yes but I just cannot OC it.
Sorry for the long answer, you may need to specify what type of machine you actually have to get a good answer.