Yep, I understand, we are in TG3 era... In fact I'm not even sure there was a TG2 free 64b version at the time (I think all the installers I have are 32b version)...
Ultimately, I did it with the 2.5.5.0 32b version on both computers. Result: 3h22m55s for the 2x p3, 12min23s for the i5. Now I can negociate!
Last time I've did this type of contest, it was with Terragen 3 Free (32b on the old, 64b on the new), but I was under the impression that TG3 was reallly (really really) slow on the p3 compare to the 2.5.5.0 version of Terragen (don't get me wrong, I think it's really fantastic that TG3 can run on a 2001 computer without crashing)... That's why, this time, I was concentrate on the Terragen version I usually use.