I think you are mixing the Intel ia64 (Itanium processor) architecture with the x64 (aka Intel EMT) architecture. The latter is often referred to as "AMD x64" - even if used on Intel processors -, because it was invented by AMD and later copied by Intel.
Both ia64 and x64 are 64bit, but are really different CPU types incompatible at the binary level, and there are different Windows 2003 editions available for ia64 and x64.
ESX does not support ia64, only x64, and obviously x64 is the architecture you are using.
According to Windows only using 7.75 of 8 GB RAM, this is something that you often see (also on physical servers). It has something to do with BIOS features or certain hardware using RAM in a way that makes it inaccessible for Windows.
With 32bit this can really be annoying. I have some DL360G5 running here that have 4 GB RAM built-in and Windows 2003 (32bit) can only use 3.35 GB.
\- Andreas
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