I don't understand why you're going for 939? I presume from the above your family don't need the performance, but considering your available budget it would make more sense to go for either A64 AM2 or Core2Duo with DDR2. A 7900GS would also have a much longer lifespan for the occasional gaming, and can be had for a bargain £120. Don't buy something you need to fully replace next time you want to upgrade, there are 2 newer platforms that should be upgradable for a long time
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If you can build systems why are you buying a dell desktop? From the Dell site it seems the XPS still uses a Pentium 4 (ugh) and Quad SLI (buggy at best)? What a waste of cash! You could build a nice Core2Duo setup and save a fair few pennies
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