Used Opterons

I just recently saw in many places that I could get Two Opteron 6172's for $100. two questions could I game well on those and why are they so cheep.

Gaming would be sub par but rendering and for use as a server they'd be perfect.

Edit: they're cheap because single core single core performance is terrible.

but would I able to Overclock an opteron

Actually yes you can overclock them, although not very far. They are based on the 10h cores that are in the Phenom II, in fact the 6172 is just two Thuban dies stuck onto one carrier. The single-core performance is going to be pretty abysmal, which makes it pretty slow for gaming. Don't get me wrong, you can absolutely play games with an Opteron, it's just not going to be much better than a $70 APU.

What would the gaming performance difference be between the 6172 and the 8350. if I am also using a 290x with a 290x

Sorry only one 290x.

About 30% less than an FX-8350. Also consider that you need a single Socket G34 motherboard (of which there are three) and ECC memory, which is slower than standard DRAM. The cheapest motherboard is $230.

What if I went duel socket I found two for $50 on ebay. Would that change much.

No, very few games can take advantage of that many threads at this time. The reason the FX8350 is faster is simply due to its IPC and clock speed. (As well as latency reduction from not using ECC as Fouquin mentioned)
If you are trying to make a cheap but powerful folding or render rig, yea sure but anything else... no.

Adding more cores only solves certain issues for particular applications. Gaming is not one of them.

A lot of cores, but games cannot use them sadly.
IPC is still better than FX 8350, but overall the 8350 is faster simply due to the high clock speed and core count.

Two of those Opterons are faster in raw preformance than a 8350, but games cannot really use it, as no games really use more than 6 threads, and most don't even use 4 properly.

Higher IPC than the 8150. Don't forget that Vishera increased IPC to be just 3% over 10h (Phenom II). These chips are simply too slow and too expensive (even at $100) for gaming.

. I have two 6176SEs paired with two 980 classies and 48GB of RAM. I should finish the watercooling loop today and have the system running. I'll report back how the performance is.

Bet Cinebench would be pretty snappy with all those cores. Can't imagine a single game that would care about having 24 cores though, or would even be able to access the second CPU considering the HT won't be polled by the OS while playing a game.

Nope. 2 CPUs in a gaming system doesn't make a difference, most games (excluding about 7 Cryengine based games) won't even be able to see the 2nd CPU.

Great... My pump didn't work. Almost burned up the CPUs before I saw it wasn't working and killed the power. Time to drain the water in the loop and install another pump. It might be a while before I get it running.

I don't think having several physical CPUs is any different for gaming than several cores.
I doubt software can tell the difference.

You're right, software alone can't tell. Windows detects more than one CPU and informs the program.

I have a small dual 771 xeon workstation that I got from work, as they were going to throw it away so why not.
2 dual cores, tested some games in it, all of the newer games used more than 2 cores, most newer ones used all cores to more or less 100%