Support for this motherboard?

I'm not sure this helps but FreeBSD 7.1 RELEASE, NetBSD 4.0.1, and OpenBSD 4.4 all support the RTL 8111B which I have on an MSI G31 mobo.
there was a thread (not ended) in current about some of those chips. they are really giving some people headaches.

I have a EP35C-DS3R and RTL 8111B and once I ran 7-STABLE on it ok.

At home I run FreeBSD 7-STABLE AMD64 on a Gigabyte MA790X-DS4 which uses the Realtek 8111B that, with some patches beyond the stable branch, seems to work "ok". Without the patches there were pretty continuous watchdog timeouts which would take the network down and up all the time with _any_ load. Interrupt usage monopolises 50% of one 2.2GHz Phenom CPU core; using polling cuts performance, such as it is, by 30%.

At work I run FreeBSD 7-STABLE i386 on a Gigabyte MA770-S3 motherboard which has a Realtek 8111C that without any patches also suffered a similar fate with watchdog timeouts taking the network offline under _any_ load at all. Since patching it, it still suffers from a bunch of "re0: watchdog timeout (missed Tx interrupts) -- recovering" errors, but at least it doesn't take the network down with it.

Personally, I'd avoid any Realtek-based NICs which have been quite notorious for flawed designs, poor implementations and general lack of credibility ever since their 10Mbps cards.

I know, I know - I should have known better, but my only available motherboard choices were Asus and Gigabyte. I started with an Asus M3A motherboard that has an Atheros/Attansic NIC which worked well, but this motherboard kept corrupting the SATA hard drive, something which the Gigabyte doesn't do despite the fact that it is using the identical SB600 Southbridge SATAII chipset.

My next NIC will be a standalone Intel Pro/1000 PT PCI-E card which Intel actually support with their own driver set for FreeBSD (including 7).
That's weird, I have zero problems with my RLT8111B other than having to run new releases to find one that supported it. I haven't applied any patches. It just works.

But a release or two back these cards were not supported. I agree with you, for now Realtek is a royal PITA. It came on my mobo so... I will also be going with the Pro/1000 next time.