I don't know anything about Slim ATX, having never heard of it. However, I do have ITX and have heard of DTX. Basically, ITX is the smallest of the standard ATX form factors which will support a single expansion slot, the DTX is slightly wider and supports two expansion slots. Of the two, the ITX seems to be the most available and it's mostly intel.
Certainly for small size and large components ITX is the route to go. My own setup consists of a Silverstone SG05 housing a i5-3570K and a GTX680. I would suggest perhaps a slightly longer case, like the SG07, if you're looking at longer cards as my GTX680 was a tight squeeze. My SG05 measures at a mere 7" tall, 8.1/2" wide and 11.1/4" deep (including the expansion card bracket on the back). Yes ,it's tiny.
- get an after-market CPU heatsink/fan and make sure it both FITS and uses a PWM fan
- make sure the motherboard has PWM fan control for the CPU
- The Corsair CX430 is known to have noisy fan issues (I bought one). If you're building a gaming rig with a good graphics card you likely want a 650W PSU anyway.
- make sure any RAM you choose isn't too tall for your CPU heatsink
- 8GB of RAM is plenty for a gamer (video or intensive photo editing may benefit from more) and 1600MHz, low-latency RAM is recommended
- whichever graphics card you use, make sure it has a good cooling solution
- *triple-check SIZE and CLEARANCE
- Windows 8 64-bit OEM, plus Start8 from Stardock is recommended
- Intel i5-3570K is the best gaming CPU
- 1155/Z77 motherboard is recommended