They should work fine together, but of course that isn't a guarantee. The speed rating is nothing more than the maximum speed the memory is warranted to work at with a high degree of stability. Since I heard nothing in your post regarding overclocking, there shouldn't be any issues to worry about. If you're planning on getting another 512MB stick anyway, you might try to match up the brand and speed to that which you're getting from your friend and use both of those in the Dell while using the older PC2700 in the HP. But since he is going to let you try it out first, you'll know for certain whether or not to buy it from him.