...doesn't suggest a power system, but more like an entry level system. Here, you can get something perfectly serviceable for $300 to $400.
Assuming the $400 budget you could:
Replace the motherboard, CPU, cooling fan, memory (If you're lucky, your new motherboard can use your old memory, but you likely won't see much improvement), and hardrive. You'll be able to save the case, keyboard and mouse, and the CD drive, but these are very cheap parts of the system. You might be able to re-install your operating system, but I wouldn't count on it, so add another $100.
You will, however, get to learn how to build your own computer: priceless!
In my home we go through a system of buying one computer for my dad, then a couple of years later one for me and so on. But last year I bought a laptop for uni instead and had intended to use my old year-2k desktop just for storage...it's pretty much on its last legs.
However, my mum has started basic IT classes and my dad's considering replacing it and giving my mum his (pretty good) '02 NEC. However, I was wondering if we could just replace key parts like the motherboard.
Current status: (from memory)
Tower pc with:
AMD-K6 3D
Two 20GB hard drives
CD player & CD RW (the latter sometimes has issues)
Floppy drive of course
Win XP Pro (Win ME too...tried to remove through a reformat, which failed)
512mb RAM
Trident Blade 3D
Pair of ok-ish speakers
17" CRT (slightly too big for the pc desk due to redecorating...no faults otherwise. My mum wants a flatscreen though)
If you're British here's the killer fact: it's an old Time. Thank goodness they're out of business.
Current usage: storage, MS Office '03, office tutorial package, recording from hi-fi/lp equipment, downloading (ltd. - wireless connection), very occasionally some gaming.

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