The reason for 4GB is that it a 64-bit OS like Home Premium 64-bit can use it.
Also, more and more programs will be able to use it. Currently Graphics and Video apps are more likely to take advantage of more memory.
Historically 4GB was unnecessary, usually.
XP can only use 3GB+ depending on the motherboard. So 4GB could still be usefully since it is usually either 2GB or 4GB of RAM possible.
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If you are tempted to max this thing out, I would look at a different computer. When you want to go beyond the specs of this PC you are talking about a better video card and a better processor. The best thing to do in that case is to go to the Dell Outlet store and search all desktops - set 2 or 3 criteria - video memory such as 512MB or 1GB, and a processor that you can live with. Then also you may be able to set a minumum amount of memory. You can get 4,6, or 8. If you want 4GB, don't even set that criteria, there are so many with 4GB RAM. A reurbished Dell is usually a new Dell that had a faoulty part or was a cancelled order. Or, if you see 15+ of the same model, they are probably coming off a business lease or a canceled business order. They have the same warranty and support as a new machine, and usually arrive in 2-3 days. Compare to a new one if possible, just to make sure there isn't a crazy sale on that model.