Refreshing older computers?

I've been running remanufactured (ebay) Dell GX520 and 620 SFF at my
business for many years with great success. I just bought a used IBM / Lenovo
ThinkCentre M58 because is seems to hold faster processors. I know these units
have seen daily use in Schools and business environments so I was wondering if
it would be a good idea to replace some of the major parts of these computers
with NOS? Power supply would probably be the most important to change or at
least clean? Probably a new Hard Drive? Motherboard? Leave it alone or
replace it? Processor? Anything else I should maybe replace? Again, I would
replace with factory spec NEW parts but probably upgrade ram to 4GB and run
the fastest processor the MB could hold. My needs are simple. 160GB. 3.0 ishh
processor. 4GB Ram. I would 2 units. One on XP and one on Win7. More parts
available for the Dell but I have found some Lenovc parts. Good or bad idea?

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