Audio Problems would appreciate help

[PrgrammingGamerNS]

I have issues with my PC's audio i would like to address. I have observed that when my CPUs usage levels hit 90-100% (I have an old CPU "core 2 duo 1.86ghz") I experience crackling audio. As a sound card deals with audio seperate from the CPU I am wondering if getting one would remedy said problem. And if so are there any PCI ones not PCI-E as the only slot in my PC is in use.

Also my CPU hits that high because one its not very good and 2 I'm a gamer, mapper, modder and programmer. The amount I use this PC it hits 100% a lot.

My PC is a Hewlett Packard a6040n desktop PC.

Upgraded to 4 gigs of ram.(even if it can only use 3.5)

Has a amd graphics card 6570.

Diamond Multimedia AMD Radeon HD 6570 2GB GDDR3 PCIE VGA

After this I would also like to ask what is the best headphones(not headsets I have a blue yeti mic) that I could get in between the 50-150$ CDN range.

[NannerBeans]

Philips SHP9500 is great. As for the crackling, a cheap usb dac/amp might solve your problem but I'm not sure if that crackling is due to interference or not. 

[PrgrammingGamerNS]

I feel like its the CPU because it only occurs at high usage.

Also thanks for the suggestion.

[PrgrammingGamerNS]

Is there any cheap DAC's/AMP's you could suggest.