For LINUX 64-bit there is REDHAT WS 3 for upto 2 CPUS, and also the AS 3 for 2 CPU +. Both have natively compiled versions for Opteron. For comparison to the P4 in 32-bit designs Id say the Opteron is better as it has higher FPU performance and shorter pipeline. As for 64-bit performance between nocona and opteron the difference in performance will be greater as the nocona is IDENTICAL to xeon with only 64-bit extensions, for the Opteron NUMA and the low latency, high BW memory architechture will kick in for designs bigger than 4GB. I shouls also note the EMT technique intel is using in the nocona is flawed for accessing the 4GB+ address space, REDHAT allready knows about this and has solved the issue but at a heavy performance penalty in memory performance.
IMO if your looking for a 64-bit workstation, INTEL is out of the game. Mentor has allready released 64-bit version of MODELSIM and CALIBRE and did not even mention or make references to INTELS NOCONA.