cpu - can be salvaged (as can its heatsink most probably)
ram - can be salvaged
graphics card - can be salvaged (although if you want a gaming pc consider upgrading?)
motherboard - (i wouldnt bother to salvage it (if you can at all) - dell?)
psu - i doubt (dell specific? although maybe)
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basically, you can take out most things - but the cpu, the ram, and the graphics are the ones you want
and then all you gotta do is buy a decent motherboard (with the correct socket for your cpu (probably 775))
buy a new pc case, buy a new psu,
put the new mobo and new psu into the new case, plug in your existing cpu, ram and graphics card into the new motherboard, and away you go
hope that answers some of your question