I have first gen P5, great cpu in its day but I wouldnt suggest anyone to buy it now. The thing is the gen after it sandy bridge had huge leaps.

1 - far greater overclock potential. Adding probably an easy 30% to clock speed, this is on top of the performance improvements native to the newer gens.
2 - automated overclocking, simply increase the turbo mode speed and thats how easy it is, and this also still underclocks cpu when idle.
3 - sataIII support on intel controller, great for modern ssd's.

ivy bridge even newer than sandy bridge also although I believe its a bit harder to overclock on that but otherwise its superior to sandy bridge, so I would get sandy or ivy bridge not first gen i5.