Hmm...I think it is possible, as the memory will clock itself lower, but that will hamper with your FSB, which'll screw around with your CPU clock speed, meaning some lost performance.
You could tweak the settings by changing the frequency of it, if your mobo supports it. You'd also will probably need to change the Ram Divider (FSB RAM) to get it working correctly. If this is a store-bought computer, like Dell/HP/Gateway/eMachines, then there is a good chance it won't work as they won't let you mess with the settings; and for good reason: An average user, unknowningly, could change some of those settings and burn out the computer, but the chances are very small that they would, unknowningly, choose a critical choice, such as CPU Voltage or FSB or maybe even memory timings. But they, the big computer manufacturers, don't want to take the chance, and anyways, it is free performance, a thing most companies don't like to advertise.
To simplify what I just said, which might've been confusing: PC2-5400 runs at the frequency of 667mhz, while PC2-4200 runs at 533mhz. These numbers, 667 & 533, are the speed in which the memory "talks" with the CPU. When you increase this number, you are increasing the CPU speed, but computers don't want that to happen because it can be dangerous if done incorrectly, so they'll throttle the memory down to what the mobo can support, which then slows the total speed of the CPU.
To even simplify that, you can try to use faster memory, but you are better off with what your motherboard can support, unless, of course, you are overclocking.
I've been browsing some forums, and have heard some success stories, so you could try it. I would first do this, install CPU-Z. Write down the entire clocks section under CPU and the Frequency and FSB RAM under Memory. Then install the memory. See what happens, especially to the Clocks section. If everything seems good, it'll be all right.