Lets do a little comparison of opterons versus A64's.
Opteron 165 = $336, 1.8GHz, 2MB Cache
A64 3800+ = $321, 2.0GHz, 1MB Cache
So far they are about equal, the cache in the opteron is made up for by the clock speed of the A64. Now overclock them to the same speed, something easy, say 2.2GHz.
Opteron 165 = 2.2GHz, 2MB Cache
A64 3800+ = 2.2GHz, 1MB Cache
Now the opteron has the lead because of the cache, now overclock them to the highest speed on air. (I'm getting the results from AT CPU and Overclocking forum)
Opteron 165 = (quoted from Opteron Results Thread)
RichUK | CCBWE 0534SPMW | 2800 | 1.62 | 44C/63C (idle/load) | TR XP90 | MSI Neo2 Plat | 2 x 512 OCZ Plat r2 PC3200 | 2.5-4-3-7 5:6 | No SS
He's a bit toasty, but do you think a A64 could get to 2800MHz?