|"Most of the original Intel P-I chipsets couldn't cache more than 64 meg of ram."|
Not an issue today, since L2 cache and cache controllers are built into the processor. Most processors can cache hundreds(?) of gigabytes of RAM. The old P3-Tualatin could cache up to 64GB. It was up to the MCH to access all of it.
I think the issue here is that either the MCH on the motherboard is limited to 32-bit DRAM access (Intel 945 and earlier), or the "memory hole" setting in the BIOS is disabled. Either way, performance shouldn't suffer, since the processor does the caching.
On my Asus board (in my signature), I had to enable "Memory hole at 15M-16M" to get both BIOS and Win64 to recognize all of my memory.
Socket 939|Dual-Core Opteron 185 @ 3.2GHz|A8N32-SLI Deluxe|4GB CL2 PC3200|SLI 8800GTS|1TB|PCIe X-Fi Titanium Pro|Win7 Ultimate x64