Just for my own info really, Opteron processors from overclockers.co.uk:

AMD Opteron model, speed	xx=14	xx=24
xx0, 1.4GHz	�182.00 (�213.85�Including VAT) (140)	�182.00 ��(�213.85�Including VAT) (240)
xx2, 1.6GHz	�227.50 (�267.31�Including VAT)	�344.95 ��(�405.32�Including VAT) (242)
xx4, 1.8GHz	�330.95 (�388.87�Including VAT)	N/A

"Opteron lines will include the 100 Series, for single-processor machines; the 200 Series, for dual-processor systems; and the 800 for computers using up to eight processors." - other than this - "number of coherent hypertransport links": 1 for the 200 series and 3 for the 800 series, I see little difference.
-Opteron's memory controller only supports ECC memory, has 128bit memory bandwitdth, has 1meg l2 cache and 128k l1 cache.
(the Athlon 64 will have half memory bandwidth and be available with L2 cache size choice of 1meg or 256k)
-According to AMD's competitive server benchmarks the single processor 246 (2.0ghz) machine has similar performance to a P4@3.2ghz on 800mhz bus (floating point) and beat both xeon 3.06ghz and P4@3.2ghz (SPECint).
there are some opteron motherboards here.

-Opteron divide by 2 multiply by 3 = pentium 4.
-Opteron scales well: more processors are not slowed down by bandwidth unlike equivalent xeons.
-Single opteron performance compared to fastest XP: eg; single MP2800 (2.225ghz) equivalent to single Xeon 2.8ghz at maya rendering. link so not much benefit to switching to single opteron. (MP2600+ �169.14inc vat, Barton XP3200+ �370.07inc vat from overclockers.co.uk)