Is the Corsair HX 650 compatible with the Antec 300?

Yes of course it's compatible i've done a couple of builds in a 300.
Most recently with a Corsair CX430V2.
The psu mounts on the bottom in a 300.
Not sure what you mean by blocked.

Thank you very much  

I see, thank you.
Do you know if the Corsair HX650w supports nVidia SLI?

CrossfireX / SLI - the Corsair HX650 doesn't care.
As long as there is enough +12V power for your cards and the rest of the system too.
It has 4 x 6+2-Pin PCI-e power leads for the graphics cards and 52Amps of +12V current.

What cards are you thinking about SLI'ing?

Dual SLI nVidia GTX 550

Corsair HX650 has you covered with plenty of power to spare.

Great! Thanks