I have an old (2004) Pentium Celeron D machine thats not doing anything I was thinking to use it just for BOINC 24/7. I think its a 2.13 GHz with 256 MB of RAM. Is this good enough for BOINC? Also, it currently doesn't have an OS but I have the OEM XP Home media that came with it or I can load Linux instead. Does it matter what OS I use in term of performance for the project?

Which project?
There's about 100 now.

(For some, 256 MB is enough, for others, just barely.)

It should be cheap and easy to find at least another 256MB of RAM for a machine like that, and it would certainly give you better general performance with WinXP and a wider choice of BOINC projects.

I have a 2.5 GHz / 512 MB laptop (XP) running SETI@home and Einstein@home (ClimatePrediction is on hold) and a 700 MHz / 192 MB laptop (NT4) running SETI and The Lattice Project (which has very seldom work for that type of PC).

Both have no problems running BOINC. I should mention, though, that I still am with BOINC 5.10.45 and 5.8.16.

Gruß,
Gundolf

[edit] There are rumors that linux benchmarks better than windows does, but I believe there's not much difference in performance when it comes to processing real work units. [/edit]

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Computer sind nicht alles im Leben. (Kleiner Scherz)