Or the IBM x3950M2 8-way Quad-Core Xeon?
Consider the following specs:
8-way Quad-Core Opteron (32 cores total)
Max Memory: 512GB (64 DIMM sockets)
11 PCI-E slots: 3x16, 3x8, and 5x4 or option for 7 PCI-E and 2HTx
Compare this with the HP ProLiant DL585G5:
4-way Quad-Core Opteron (16 cores total)
Max Memory: 256GB (32 DIMM sockets)
7 PCI-E slots: 3x8, and 4x4
Aside from 8 Quad-Core processor sockets, the significant differences are 64 DIMM sockets, doubling the maximum memory of the 4-way, and 11 PCI-E slots (depending on the actual architecture, this could be 92 PCI-E lanes!)
Some pricing from the HP web site below.
w/4x2.2GHz $10,389, 4x2.3GHz $12,189, 4x2.5GHz $16,189
64GB 32x2GB +$4,495, 128GB 32x4GB +$13,455, 256GB 32x8GB +$55,863
w/4x2.2GHz $16,973, 8x2.3GHz $27,291, 8x2.5GHz $46,891
128GB 64x2GB +$7,612, 256GB 64x4GB +$26,492, 512GB 64x8GB +$75,316
8-way systems were somewhat popular in the Pentium III Xeon/ProFusion days, but Intel did not follow it with a respectable (read on when you stop laughing) 8-way chipset for the NetBurst based Xeon processors. HP did their own chipset for the DL740(?) which they considered moderately successful (in that it was profitable but did not warrant continuation for the next generation dual core processors).
The ProLiant DL785 posts a respectable TPC-H benchmark result of 52,860 QphH@300GB (SQL Server 2008), compared with 46,034 for the 8-way IBM x3950 with 8 Xeon 7350 (SQL Server 2005). The ProLiant DL585G5 also posted a top 4-way SQL Server TPC-C result of 471,883tpm-C on the AMD Opteron 8360 at 2.5GHz. I was really expecting that the AMD quad-core needed to be at 2.8GHz to reach this performance level.
Now getting an actual database (SQL Server or any other) to scale to 16 or 32 cores is not a simple matter. I do suggest conducting a proper quantitative scaling analysis, i.e., measuring maximum throughput with 1, 2, 4, 8, 16, and 32 cores.
The other reasons for going with the 8-way is the extra DIMM sockets. The 4-way QC 2.3GHz DL585 with 256GB memory (32x8GB DIMMs) is $68K, versus $54K for the 8-way DL785 with 64x4GB DIMMs (it is necessary to populate all 8 processor sockets to get 64 DIMM sockets,
but one could restrict SQL Server to 4 sockets if per processor licensing is involved, see Andy's comment below). Even at 128GB, it is $26K for the 4-way DL585 with 32x4GB DIMMs versus $35K for the 8-way with 64x2GB. The extra 16 cores for a $9K delta is highly attractive in CAL situations.
One would also think that the 11 PCI-E sockets could support phenomenally high sequential disk transfer rates: 800MB/sec per first generation PCI-E SAS RAID controller, 1,100MB/sec+ for second generation (in x8 slot). But there is no published data on this for the AMD systems with PCI-E. The HP Itanium systems can do over 15GB/sec in SQL Server table scans.