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#11
Almost any machine on sale is good enough to play music on - and depending on which Sims it is your kids play, most should be quite capable.
Take a look at the sale items currently - around £350 would get you a laptop that will pretty much anything you need. The same money on a desktop (main box only - no screen or mouse/keyboard) should be perfectly good enough.
just picking one of each at random to give you an idea....
Zoostorm Premium PC Core i3-2120 6GB DDR3 500GB W7HP (7877-0094) - dabs.com (personally, I think that is a little overpriced)
Fujitsu LIFEBOOK AH531 Core i3-2310M 2.1GHz 4GB 320GB W7HP (VFY:AH531MR3A2GB) - dabs.com
Your current system has about 170GB in use.
Beyond that, you should ask around and see what's available before the January sales end, and Windows7 gets to be a rarity - I personally consider Windows 8 to be optimised for touch-screens, and awkward for mouse/keyboard control, at least until you get to play with it for a fair while!