Graphics Card Upgrade

PSU:

Internal 600 Watt (100V-240V)

Form Factor: internal ATX
Total wattage: 600W
Nominal input voltage range:
- 100-127V (50-60Hz)
- 200-240V (50-60Hz)
PCI-E power connector (for high-end video cards): 2
Dimensions: 150mm x 140mm x 86mm (5.9 x 5.5 x 3.4 inches)
* This power supply has an LED to indicate a possible failure condition when LED is off and power is connected

Case:

GPU:

orm Factor: Full-height ATX bracket
Dimensions: 9.9 cm x 14.4 cm (3.9 in x 5.7 in)
Maximum resolution:
- DisplayPort resolution: 2560 x 1600 at 60Hz
- DVI (dual-link) resolution: 2560 x 1600 x 32bpp at 60Hz
- HDMI resolution: 1920 x 1080
- Analog VGA resolution (with adapter): 2048 x 1536 x 32bpp at 75Hz
  NOTE:
  Not all ports support the highest resolutions.
4 GB onboard memory
Memory interface: 128-bit DDR3
Total power consumption: ~ 50 W max
Supports Blu-ray
Supports HDCP
Supports up to two displays at the same time with independent resolutions up to the maximum , in the following configurations:
- DVI(VGA)+HDMI +DP
Above are the current specs of my computer. Which is the HP Envy Phoenix 810-160 which is a great machine but I noticed the graphics card has had a harder time keeping up to the rest of the computer. So I would like to upgrade the graphcis card. I was looking at the GeForce GTX 970 from the measurment I took it looks like it would fit in the case. My questions are would upgrading the GPU void the warranty? Is this the best GPU to upgrade to? Is the PSU powerful enough to power the GTX 970 or would I need to upgrade that too? Also how would I do the upgrade in respect to the drivers I heard something about Legacy Mode?

Thanks