Just doing a rough comparison check between the stats that are listed for the Arctic Cooling Freezer 7, your height and depth dimensions (are these internal dimensions?) appear to be too small for this aftermarket cooler. You will have to either find another aftermarket cooler to fit within the dimensions of your Inspiron 518, or do some serious modding of both the heatsink and the case itself to make the heatsink fit. To answer your other question you could remove the old heatsink (I would imagine the stock cooler also uses push pins as a mounting method) and simply slap the new heatsink into place, without removing the motherboard. It is all a matter of making sure the aftermarket cooler (whatever you decide on) fits within the tight space in the case.
From what I see (after looking online at a review site) there may be enough room inside the case for an aftermarket cooler as long as it doesn't run into the power supply. The following link is a heatsink that may fit inside the case without bumping into the power supply. It is rated for Core 2 Quad and Core 2 Duo, so you should not have very many problems with overheating.
http://www.newegg.com/Product/...x?Item=N82E16835103039
I am curious to see how this works out for ya.
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