So I tried the adapter settings mentioned earlier to no avail.
I also received the following email from Marvell, which confirms the above post.
We have noticed this problem with several Asus + Nvidia based chipset + Vista/Windows 7 x64 + 4GB memory. Looks like there is some compatibility issue with this combination. Unfortunately when this motherboard was released, there was no Vista or Windows 7 on the market to test. There's only a few solutions for this problem.
1. Use less memory.
2. Use a 32-bit O/S
3. Install an add-in NIC
So back to my initial question. . . Can anyone recommend a 10/100/1000 NIC that they can confirm is working in Windows 7 x64 with over 4GB of memory?