r7 260x with 300W?

Solution
Get a new power supply and the r260x (what I did)

or

Get a GTX 750 Ti but not the ACX FTW version (what I couldn't do since my PSU maxed out at 240 watts but the PSU in the ASUS is probably a better quality one compared to my old one)
The PSU included in the ASUS M11BB only has a maximum combined +12V current rating of 18 Amps.

The Radeon R7 260X requires one 75 Watt 6-pin PCI Express supplementary power connector that the PSU doesn't have.

Look for an NVIDIA GeForce 750 Ti card that doesn't require a 6-pin PCI Express supplementary power connector.

If you compare an NVIDIA reference design GeForce 750 Ti to an AMD reference design Radeon R7 260X the GeForce 750 Ti has a 15% performance advantage over the Radeon R7 260X.
 
Get a new power supply and the r260x (what I did)

or

Get a GTX 750 Ti but not the ACX FTW version (what I couldn't do since my PSU maxed out at 240 watts but the PSU in the ASUS is probably a better quality one compared to my old one)
 
Solution