The important thing to know is if 450W is continuous power or peak power, and, just as importantly (In my opinion) the efficiency of your PSU (Efficiency ~= quality).
A 6750 and a 2300 won't eat up 450W even at their peak. That being said, a PSU is most efficient at around the 50% load mark (give or take). The closer you get to very low or very high loads, the PSU loses efficiency, which has the possibility to be bad if you start with a low quality PSU. You can stop reading now if you like.
There is also an issue with which rail that power is coming on. The side panel of the PSU will tell you that information. Basically, not all the power (450W in this case) is from a single source (a rail). Some of that is for the CPU, motherboard, and then other components (like the GPU), so be aware of this.
Last but not least, I am knowledgeable, but not an expert, so proceed at your own risk.