Nic's: 2 x Realtek 8139 10/100 M/Bit
Hooray, another (probably) easy one!
Throw out the Realteks and use some decent NICs (Intel is always a good choice). From the FreeBSD rl driver source code:
* The RealTek 8139 PCI NIC redefines the meaning of 'low end.' This is
* probably the worst PCI ethernet controller ever made ...
Personally, I've found them not to be quite that bad with other operating systems, but it's a well known fact that they don't perform very well with FreeBSD.
Your P3 500 MHz CPU should be able to handle *way* more than 12 Mbps with good NICs.