Only the very stoutest 300W supplies will run something like you propose (sparkle and HEC). Chances are good the machine will operate, but the supply will blow very hot air showing how highly it is loaded. Of course side effects might range anywhere from nil to loss of stability or even re-boots. Since the term "300W" can be applied to about any supply it's hard to make a judgement on the advisablity of running your rig off of the current supply, but I would be prepared to upgrade the unit. Fortunately directron has a large range of supplies that are less than $100 dollars that are sure to please. Consider the 350W FSP as a practical minimum for your needs. It is a mere $38. It's output has been tested in excess of 450W, and there are even beefier unit available for 60-70 bucks.