First time building a gaming PC, advice pls!

Ok so before you all laugh at what my current specs are for the gaming pc I have and I know they are ancient but this PC that I've had for close to a decade has been good to me for quite a long time and did what I needed it to do. It still runs great but it's due for a drastic upgrade since it's not as fast and smooth as it used to and with current games needing optimal specs to run games it's time I try and build my own. I spent a lot of money when I first got it from Alienware I think it was around $1700 and really had no clue that with that amount of money I could've built a better PC but I was young and clueless. I still am a little new to this but I am here asking for your help. What I would like is a PC that has a lot of Memory. I am not an extreme gamer but I do game mostly mobas and fps. The reason for memory is I am also a DJ and have quite a bit of music and will be using this PC to load my music. I need this new PC to be fast as well and can run games at optimal settings if I chose to. Any recommendations would be appreciated. Budget is between $1000-$1500. I want a PC that will keep on running smoothly for a while. Also would I require a new case because the one I have is huge.

Without further a do my specs currently lol:

Area-51® 7500

Processor: Intel® Core™ 2 Duo E8400 3.00GHz 6MB Cache 1333MHz FSB

Alienware P2 Chassis: Alienware® P2 Chassis with AlienIce™ 3.0 Video Cooling - Space Black

System Lighting: Alienware® Standard System Lighting - Terra Green

Power Supply: Alienware® 750 Watt Multi-GPU Approved Power Supply

Graphics Processor: NVIDIA GeForce 650ti

Memory: 2GB Dual Channel DDR2 SDRAM at 800MHz - 2 x 1024MB

Motherboard: Alienware® Approved NVIDIA® nForce 680i SLI Motherboard

System Drive: Single Drive Configuration - 250GB SATA 3Gb/s 7,200RPM 16MB Cache

Sound Card: High-Definition 7.1 Performance Audio

Additional Monitors: 19" Samsung 953BW – 1440 x 900 (2ms) Widescreen Flat Panel



And this what I am thinking of getting:

Processor: Intel Core i5-6600K 3.5GHz Quad-Core

CPU Cooler: CRYORIG M9i 48.4 CFM

Power Supply: XFX PRO Black Edition 850W 80+

Graphics Processor: currently going to stick with my GeForce 650ti until the 1070 comes out.

Memory: Crucial 8GB (2 x 4GB) DDR4-2133 Memory

Motherboard: Gigabyte GA-Z170X-UD3 ATX LGA1151

SSD: Crucial BX100 250GB 2.5"

Hard Drive: Seagate Barracuda 1TB 3.5"

I need your thoughts on the mobo,cpu, ssd, and the hard drive. Are there better options? Also the case i currently have (Alienware7500) is huge but i am not sure if all of these new parts would fit or if i should buy a new case. I also chose the i5-6600k 3.5GHz to have the option of overclocking in the future but i am still relatively new at it so not sure if i should go with a different CPU. Thanks in advance for the help.