Hi,
I'm the mxge(4) author. A bit more information.. The 8A-C was one of the first 10G cards we made. As such, it is slower for packet processing and not compatible with our newer "sniffer" packet sniffing software. Also, if you are buying it used, you may need to update the firmware. The earliest 8A NICs (circa 2006) require an EEPROM update to support MSI-X (required for multiple tx/rx queues). You want to download our "10G" toolkit for FreeBSD. We ship it in a lowest-common-denominator 32b format, so you'll want the 32-bit compat stuff installed if you're on amd64.
Best regards,
Drew