Looking for one of the Upgrade out of SSD vs RAM vs CPU Cooling vs Thermal Paste

Hello !

I have System config as Follows:

Processor: Intel(R) Core(TM) i5-4570 CPU @ 3.20GHz (4 CPUs), ~3.2GHz
RAM : 2X4 GB Corsair Vengence @ 1600Mhz
HDD : Seagate 2TB Desktop Internal Sata Drive (ST2000DM001)? one more 320GB HDD from Dead Laptop temporarily connected.
Cooler : Stock Intel Cooler
Thermal : Mostly ABSENT (The guy who assembled forgot to bring paste & later on I forgot about it)
Power Supply: ANTEC VP650P
Motherboard : ASUS B85-G
Cabinet: Cooler Master HAF 912 with Default fan config(1X Front (IN), 1XBack(OUT))
GPU : Overclocked Intel 4600 @ 1500Mhz
OS : WINDOWS 7 Ultimate 64bit

USAGE:
This Rig is Mainly Built for Gaming. I am Saving for GTX 970.
Also, I use this Rig for Photoshop and Coding (Netbeans).

As summer is approaching ambient temps are around 30-40*C.
CPU Temps Under Idle are 37-39*C
CPU under Gaming is about 45-55*C ( Dragon age : Origins,can try CRYSIS 3 if required)
CPU under Benchmark 70*C+( I closed the benchmark at 70*C)

As temps were decent for overclocked GPU (except During Benchmark) I didn't pay attention to the thermal compound. (I didn't try to apply it on my own because last time I tried I killed my Laptop).
I have programmed coolers to reach to 100% if 45*C is crossed

I get about 50% Free RAM with 5-6 Tabs on Chromium.

I can buy one 120GB SSD OR 4GB RAM STICK N COOLER if Required.

Question:
1.I would like to know if only by applying thermal paste will have any major impact (5-10*C) in CPU cooling?

For Now System Stability > System Performace but again, it depends on Impact if i get great bang for the buck by investing in performance upgrade I will prefer that.

Cooling Option : Cooler master Hyper 212X CPU Cooler or CHEAPER
Thermal compound: Deepcool Z5 Tharmal Paste

2. Kindly Suggest me what will better for my system.
SSD or RAM or Cooling

3. Will I be able to install Hyper 212X in my case ?( I should be, because it's big enough, but if not please correct me).

4. Will Hyper 212X cause any space issue with my RAM slots or GPU slot. ?
Considering i will be adding more 2x4GB RAM in the future and A GTX 970

Few More Points to Consider:
*I won't be Overclocking CPU as its locked.
*I will Disable iGPU once i get my GTX 970 (if that reduces Heat).
*GPU will genrate Heat that should be considered.
*The bootup time is around 30-40 sec which is not bad I guess and am happy with the speed at which programs load.
*I am from India so few things are ridiculously costly here.

Please let me know if any more details are needed
Thank You !