I'm currently mining with 2 GPU's. My XFX RX580 8GB sits at 90c while my R9 280x sits at only 74c.

Both are running stock. Mining ETH with 280X I get about 13.3 mh/s and with RX580 I get 22 mh/s.

My system:
Gigabyte Z87X-UD3H ATX LGA1150 Z87 DDR3 3PCI-E16 3PCI-E1
Intel Core i5 4670K Unlocked Quad Core 3.4GHZ
Kingston SSDNow V300 240GB 2.5in SATA3 LSI SandForce SSD
G.SKILL Ripjaws X F3-14900CL9D-8GBXL 8GB 2X4GB DDR3-1866
Seasonic G Series 750W ATX 80PLUS Gold Modular 120mm PSU

MY GPU's
R9 280x and RX 580 8GB on the way

I will be using this computer mainly for mining running 24/7 and some light web surfing. Should I be worried with the PSU I have?

Thanks in advance.