Welcome to Tom's Forum!
The DDR3-1333 kit will run at a reduced Frequency (1066 or 800 MHz) by default so it's fine to run the kit.
MSI G41M-P26 - http://www.msi.com/product/mb/G41M-P26.html
- Supports FSB 800/1066/1333 MHz
- Supports two DIMM DDR3 800/1066/1333*(OC) DRAM
To remedy the 80%
try running MSCONFIG and turn-off the Startup items (Disable All), and also Services [x] Hide All Microsoft Services ... then deselect the unneeded items, click OK and reboot.