Differences bettween ATX, M-ATX, and M ITX

Well here is the thing, Are you going to be using DUAL SLI OR DUAL CROSSFIRE? Are you going to be using internal ssd or internal HD? If not then M-ATX is good for you, But I always suggest going with the ATX.

What will be the main use for this pc?
Gaming, or apps like rendering?

For a budget build, I like to recommend that one builds for future expandability.
That means paying a bit more up front for some parts that allow for an easier future upgrade.
Let me start where you might not expect:
1. Buy a good 620w psu. Such a unit will run any future graphics card.
I would normally suggest Seasonic 620w:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817151096
But this EVGA 600w unit is going for a very good price:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817438014
2. Buy a Z97 based motherboard. Z97 will allow you to install a overclockable cpu and even offer a future 14nm broadwell upgrade.
You should fine one for about $100.
Here is a M-ATX : http://www.newegg.com/Product/Product.aspx?Item=N82E16813157521
3. I suggest a G3258. It is a overclockable dual core at a budget price of about $75.
Here is what it can do: http://techreport.com/review/26735/overclocking-intel-pentium-g3258-anniversary-edition-processor
In time, you can upgrade to any cpu that you want and market the G3258.
4. The intel stock cooler will do the job up to a point. But, I suggest a $30 tower type cooler like the cm hyper212 with a 120mm fan. It will cool better and be quieter under load.
5. For ram, speed is not important. Buy a 8gb kit of 2 x 4gb DDR3 1.5v ram.
6. Cases are a personal thing. Buy one you love. Most will do the job for <$50.
It would be hard to beat $34 delivered for this Antec GX500 :
http://www.newegg.com/Product/Product.aspx?Item=N82E16811129191
7. The graphics card is the most important component for gaming. My usual rule of thumb is to budget 2x the cpu cost for the graphics card. I like the GTX750ti and EVGA as a brand.
Here is a superclock version:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814487024
You could go stronger in the video card if your budget permits and your games need it.
On the other hand, you could build using the integrated graphics and see how you do.
By deferring on the graphics card, you will get a better idea of what you really need.
Integrated is fine for sims, but not fast action games.
8. Lastly, I will never build again without a SSD for the "C" drive. It makes everything you do so much quicker. 120gb will hold the OS and a handful of games. With 240gb you may never need a hard drive at all. Defer on a hard drive until your ssd approaches 90% full.

-------------good luck------------

------------Stock rant on 8 core FUD--------------

I have heard some say that 8 cores will be required for future games.
I think that is FUD perpetuated by AMD.
Game developers want the largest possible market for their games.
No game developer will willingly undertake the extra cost to make their game multi core enabled and also require many cores to run.
They would not sell many games.
Most games today only use one or two cores.
There are a few exceptions, FSX is one.
It is more important that the cores be fast.
AMD hates that because their cores are much less efficient than intel's. Perhaps 30% slower per clock.
That is also a motivation for mantle, a technology that improves the efficiency of graphics drivers.
Mantle is most important for slow chips, but is irrelevant for $200 class intel cpu's.
Just because you see activity on windows task manager across all cores, do not assume your job is using all those threads.
What you are seeing is windows spreading the activity across all available threads.
Then there is "Amdahl's law" which limits how many threads can be useful, depending on the speed of the main thread.
Today, a I5-4690K is as good as it gets for gaming.
I see many reviews from pleased users switching from a FX-8350 to a i5-4670K.
I see none who are pleased switching from a i5-4670K to a FX-8350.
The only reason for a i7 4790K compared to a i5-4690K is if the $100 difference is not important to you.
For your $100, you will get a better binned chip and some extra L3 cache.
The extra hyperthreads will not be very useful to the gamer.
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No doubt a fast dual core like the G3258 might have a problem in some games.
The report above had a higher oc than I would feel comfortable with. But, 4.4 should be very reasonable.
If you know that the games you play are both cpu dependent and multicore enabled, then an intel quad would be appropriate.
But, today it is half the cost of other options.
With G3258 you have options. With AMD slow cores, you don't.

For gaming, a i5-4690K is as good as it gets, and is likely to be for 3-4 years.
It certainly is a viable plan to buy a strong cpu up front and upgrade graphics later.
If you live near a microcenter, they will sell you one for $200.
As it happens, they also have a G3258 and Z97 motherboard bundle for $99.
http://www.microcenter.com/site/brands/G3258Bundle.aspx

I see graphics upgrades as being more likely in the future.
Today, you need dual strong >$500 cards for gaming on triple monitors or the upcoming 4k monitors.

If you buy a 4690K up front, and a $100 graphics card(the slowest that is remotely suitable for gaming, you will be out the $100 for the graphics card when you buy the $250 graphics card upgrade.
I would use the integrated 4690K graphics for a while and not waste $100 on an interim graphics card.

Conversely, if you buy a Z97 motherboard, G3258, and a $250 graphics card, you will be out only the G3250 when it comes time to upgrade the cpu.

Christmas is a long way off. I tend to be impatient.
A GTX760 is a $250 card, A GTX770 is a $310 card. Not really that much difference in performance.

And... for most games A G3258 and a 4690K will be limited more by the graphics card than the cpu.

Actually, I think I would go slow and use a G3258 without any discrete graphics card at all initially.
You will get some experience with overclocking and the strategy games will play just fine.
You can then add the graphics capability that works best for you.

Look for some G3258 reviews and benchmarks which are just now coming out. It is a surprisingly capable chip. Graphics cards are constantly being updated with better price performance and environmentals. By Christmas, we may well see some attractive graphics offerings. That is strictly speculation. Anybody that says they know doesn't.

15% true enough. But the difference is close enough that you will not tell any real difference in actual usage. It takes a synthetic benchmarks to quantify that.
That is why Tom's shows the two cards on adjoining performance tiers.

Bottom line is that you mostly get what you pay for.