If I crossfire an r7 265 will a M5A97 R2.0 MOBO support it? Yes
Supports AMD Quad-GPU CrossFireX™ Technology
Expansion Slots
1 x PCIe 2.0 x16 (blue)
1 x PCIe 2.0 x16 (x4 mode, black) <---- should be OK with R7 265
And is a 750 watt PSU enough? Yes
620W is enough. As long as its a good quality PSU - per tiers 1,2 or 3 of
http://www.tomshardware.com/forum/id-2547993/psu-tier-list.html#xtor=EPR-8886
Will my FX 6300 bottleneck? - Generally NO.
But if you try playing a game that asks for an i7 or such (eg Watch Dogs) then expect it not to work real well.