My question is why 939 over 754? What are the actual benefits?

Simply put, there are three clear advantages to using socket 939.

The first, Socket 939 can utilize dual-channel ram, which will yield a performance gain over Socket 754.

The second, Socket 939 AMD Athlon 64 .09 Micron Processors (3200+ and 3500+ are .09 Micron) run VERY cool. Over the weekend, I built a 3200+ Socket 939 system for a client. The system ran at 20 Degrees Celsius! Just for comparison, my older .13 Micron 3500+ runs over 36 Degrees Celsius under load...

The third, with the new nForce4 chipset, which is only for socket 939, you can run the latest in graphics cards, with its PCI-Express support, as well as SATA-II (300mb/s) hard disks.

A most important reason for choosing 939 over 754:
754 is dying. AMD will not release any more desktop cpus for 754 (except maybe another sempron? Sempron aren't really cpus.. )
There will still be mobile cpus for the 754 but these won't be usable on a desktop unless you have a motherboard and bios that lets you use mobiles. (DFI Lanparty 250Gb lets you do this.. unsure of other boards).
If you want to upgrade in the future, 939 is the way to go. All the new AMD cpus will be coming out on 939, including the dual cores to my understanding, so a NF4 motherboard looks like the way to go to allow for upgrades in the future.
Socket 754 is all good, but the single channel memory controller can be a killer if you are trying to overclock two sticks of ram. (Not sure if this is an issue for you.)

So if I go 939 I am looking at these boards.
Chaintech VNF4 Ultra zenith
Gigabyte GA-K8NF-9
Asus A8V Deluxe

There are other boards too, off the top of my head I know that MSI and DFI make them.

In my first post, I talked about a machine that I built for a client. That machine had the Chaintech VNF4 Ultra. It worked just fine, however, I'd always be more comfortable buying an MSI or an Asus motherboard.

One other reason to go with the 939 is that the chip has built in virus protection. Or so I was told. As far as the board, I would go with MSI.

There is no chip with built in virus protection. That is an advertising ploy by amd and they are getting a lot of flack on it. The virus protection comes from service pack 2. The 754 is sited to have this also.

I think the "virus protection" is from a built in firewall on the nForce 3/4 chipsets.
Never use it myself though..

the antivirus stops only a limited number of viruses mainly ones harmful to your cpu...it stops any packet overflow virus, and intel's are coming out with the same thing in the very near future. Its not a gimic, it actually works, just it is not a full blown virus protection.

I have a 754, i knew when I got it I would have limited upgradability.

But it doesn't matter to me so much. My next upgrade will be a new board and new chipset again anyway.