The 6100 is still $20 more than the FX-6300. Skylake boards are more expensive too. I can fit an FX-6300 and an R7 370 in a $500 budget, with the i3 I'd have to downgrade to a 750Ti or R7 360. No. Don't touch the BCLK unless you really know what you're doing. Base clock OC is a bad idea. Also, it's Z170, Z97 is for Haswell/Broadwell.
It's a bit over $500 with the i5-6400. With the i3, it'll be ~$40 under. http://pcpartpicker.com/user/Funky7Monkey/saved/6dYhP6
Except if you really have the money. You CAN still take an i7, just don't if you have to choose between a GTX 980Ti and an i7. GPU is more essential. That could be quite a headstart for AMD.
Bad idea. A C2QX will be at least 7 years old, they're also not cheap - unlike C2D/Qs, they're enthusiast CPUs, they don't depreciate as quickly and were very expensive at release. For the money you're probably better off getting a new H110/81 board and Skylake/Haswell i3, though if you want anything remotely future proof you should get an i5-6400. Edit: Just checked, you can get a QX9750 for $120 - same price as a new Skylake i3, a new board is $50. The i3 will perform much better and the QX has most likely been overclocked/overvolted, might not last too long.
Pretty sure you're the one who needs to do more research. And choosing a CPU is not a matter of opinion, a 7 year old CPU is objectively worse than a new one, and not worth buying.
Then you should know buying outdated hardware is never a good idea. Here have some benchmarks: http://www.cpubenchmark.net/compare.php?cmp[]=1033&cmp[]=2617 That's a QX9770 - Top tier CPU at time of release, priced at $1400, now still sold for $300 - getting completely destroyed by a Skylake i3. I'm not here to argue with you - I'm trying to help people. I've been building PCs for years. I know what I'm doing. And I actually research and check my sources before posting. Please don't post advice if you clearly have no idea what you're talking about.
Then prove it. I just provided benchmark scores. I can give you links to the parts if you're too lazy to search Ebay and PCPP for them. I can give you many reasons why buying an old CPU is a bad idea. You have nothing to back your point - not even an argument other than "I'm right because I know more than you because ???". I'll also ask that if you do decide to continue this argument, you take it to PM. We're cluttering the thread.
Even I wouldn't buy a Q2E / C2Q for any kind of modern gaming (And that's me! "Cheap used parts whore Galore!"), it's just silly, especially when you consider most games recommend a I5 2500k minimum, not only that but they really, really, are overpriced. And BlueScreen isn't a know it all, he's a know it all who knows his shit.
Although, for a while, I actually was running beamng on my "fabulous pile of steaming crap," which had a core 2 duo from... '08 I believe? Core 2 Duo E8500 @ 3.2 ghz, 4gb of DDR2, and a Zotac 9800GT 512mb. It will run (mostly) on crap, but forget about using more than 1 vehicle or decent settings. However those CPUs are like $10 so if you simply must play but are living on the streets or something... It will get the job done (usually). However, I say this with an enormous caveat- a system like that simply can't run the larger maps due to a lack of both VRAM and DRAM. All stock maps except East Coast and Hirochi Raceway worked, as for mods, your mileage will vary. I am NOT recommending this, I am just saying it will work