What PSU to take for my PC?

You can get any Gold certified PSU from Corsair, EVGA, NZXT if you want a top one.

The GTX 980 Ti require a quality 600W PSU that can deliver 38Amps or more on its +12V rail and has a minimum 2 x 8-pin PCI-E connectors.

Consider this:

PCPartPicker part list / Price breakdown by merchant

Power Supply: EVGA SuperNOVA GS 650W 80+ Gold Certified Fully-Modular ATX Power Supply ($75.98 @ Newegg)
Total: $75.98
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-07-24 08:56 EDT-0400

The EVGA GS 650W is a 80+ Gold Certified Fully-Modular ATX "Tier 1" quality PSU that can deliver a mammoth 52Amps on its +12V rails, comes with 4 x 8-pin connectors, which is more than enough to power your entire rig.

Update:

Basic key differences marked in RED

Key Feature:

120-G1-0650-XR / 220-GS-0650-V1 / 220-G2-0650-Y1

Wattage 650 650 650
ATX12V/ESP12V 10-5A 12-6A 10A
Modular Full Full Full
Continuous Power Yes Yes Yes
Single/Multi Rail Design Multi Single Single
MTBF (Hours) 100,000 Hours 100,000 Hours 100,000 Hours
Compatible with SLI/Crossfire Yes Yes Yes
Operating Temp Range (°C) 0° to 50° C 0° to 50° C 0° to 50° C
Warranty (Years) 10 5 7
AC Input Specifications
Input Voltage (VAC) 100 - 240 VAC 100 - 240 VAC 100 - 240 VAC
Input Frequency (Hz) 50 - 60 Hz 50 - 60 Hz 50 - 60 Hz
Active PFC Yes Yes Yes
Efficiency 90% (115VAC) / 92% (220VAC~240VAC) or Higher 90% (115VAC) / 92% (220VAC~240VAC) or Higher 90% (115VAC) / 92% (220VAC~240VAC) or Higher
DC Outputs Specifications
+12V (A) 20A / 20A / 20A / 20A 54A 54.1A
+12V (W) 636W 648W 649.2W
+3.3V (A) 25A 20A 24A
+5V (A) 25A 20A 22A
Combined 3.3V + 5V (W) 150W 100W 110W
+5vsb (A) 3A 2.5A 3.0A
-12V (A) 0.8A 0.3A 0.5A
Cooling System
Automatic Fan Yes Yes Yes
Fan Bearing Type Double Ball Bearing Teflon Nano-Steel Bearing Double Ball Bearing
Fan Size (mm) 135mm 120mm 140mm
Efficiency Ratings
80 Plus Certification Gold Gold Gold
ErP Lot6 Compliance with ErP Lot 6 2013 Requirement Compliance with ErP Lot 6 2013 Requirement
Power Protections
OPP (Over Power Protection) Yes Yes Yes
UVP (Under Voltage Protection) Yes Yes Yes
OVP (Over Voltage Protecion) Yes Yes Yes
SCP (Short Circuit Protecion) Yes Yes Yes
OCP (Over Current Protection) Yes No Yes
OTP (Over Temperature Protection) Yes Yes Yes
Safety Certifications
TUV Yes Yes Yes
cUL Yes No No
UL Yes No No
CB Yes Yes Yes
CE Yes Yes Yes
FCC Yes Yes Yes
C-Tick Yes Yes Yes
cTUVus Yes Yes Yes
BSMI (Taiwan) Yes No Yes
CCC (China) Yes No No
WEEE Yes Yes Yes
RoHS Yes Yes Yes
Output Cables (Totals)
Total Cables Included 12 10 10
Sleeved Cables Sleeved Sleeved Sleeved
Motherboard Power Cable 1x 600mm (m) 1x 600mm (m) 1x 600mm (m)
EPS 12 Cable Length 1x 600mm (m), 1x 750mm(m) 1x 650mm (m), 1x 550mm+550mm (m) 1x 700mm, 850mm (m)
PCI-E Cable (VGA) 2x 600mm(m), 2x 750mm(m) 2x 500mm, 600mm (m) 2x 700mm, 850mm (m)
SATA 5 pin 1x 450mm, 550mm, 650mm (m), 2x 550mm, 650mm, 750mm (m) 1x 550mm, 650mm, 750mm, 850mm (m), 1x 550mm, 650mm 3x 500mm, 600mm, 700mm (m)
Molex 4 pin 2x 550mm 1x 550mm, 650mm, 750mm, 850mm (m) 2x 500mm, 600mm, 700mm
Floppy Connector NA 1x 100mm 1x 100mm
AC Power Cable 1x 1800mm (m) 1x 1500mm (m) 1x 1500mm (m)
Misc Details
Power Supply Dimensions (H x W x L) 85.00 x 150.00 x 180.00 86.00 x 150.00 x 150.00 85.00 x 150.00 x 165.00
Shipping Box Dimensions (H x W x L) 266.00 x 127.00 x 330.00 11.00 x 7.50 x 13.00 266.00 x 127.00 x 330.00
Weight 8.00 5.09 6.30
Case Design Black Texture Black Texture Black Texture
ECO Mode No Yes Yes

Since the GS lack "Over Current Protection", I suggest you get cost effective G1:

PCPartPicker part list / Price breakdown by merchant

Power Supply: EVGA SuperNOVA NEX 650W 80+ Gold Certified Fully-Modular ATX Power Supply ($69.99 @ NCIX US)
Total: $69.99
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-07-24 09:19 EDT-0400

The G1 can deliver a max output of 636W with a quality 53Amps on its +12V rails combined, which is more than enough for your config.

No need to worry about Single vs Multi rail. Demonstration by PSU expert Jonny Guru himself in this latest video:
[video="https://www.youtube.com/watch?v=PWtKSHT2od8"][/video]

Cheers!

I believe the G1 models are multi rail where the G2 and GS are single.

GS have a 5 year warranty, G2 is 7 years and G1 is 10.

The G2 and G1 units both have a 10 year warranty. The B2 and B1 (aka NEX B) are only 5. All the "2" units are higher grade, single rail as you said, have larger fans, and are slightly more expensive. "2" units are made by Superflower, where "1" units are made by FSP.

To the OP I would consider the 750B2 or G2 if you want fully modular and/or more efficient. The B2 is semi-modular, but you will use almost all the non-modular cables, and in tests is almost as efficient as a silver. The G2 is 100% modular.

That might be a good general rule but it's not 100% accurate. For example, the 1000W G1 only has a 5 year warranty.

That said, thanks for the details.

Cheers!