thats a bit of an old pc like the one im using now, old but serviceable.
Is it an AM2 mobo or 939? then its either a brisbane or toledo cpu worth a check, the brisbanes were the better cpu.
im guessing its am2.
The 4800 cpu's seems to hold their price, so cpu+working mobo and ram , 2nd hand price around £70.
It would do as an upgrade for a lousy sempron / old celeron user, include the graphics card & your upto £100 '+' mark, do some one a favour on an upgrade and charge a service fee working another £30 into it.
Eitherway the hard drive is the limiting factor as pointed out.
If you resell replace it with at least a 160 gig drive they are cheap around £30 , clone / reinstall your software across.
The 40 gig lifespan is questionable ,unless you can vouch for it.
With a flat screen pc worth upto £200 as a reasonable desktop , depends what the case condition looks like, anything chipped worn, forget it, some people buy on looks.
As is tops £130 + - a few quid.
Dont forget you got it for free, hey do your dad a favour!
pass it on.