Intel didn't do idle power gating until Nehalem, so its not surprising that your Core 2-era processor idles quite high. Newer Intel generations are even more power efficient at idle, with Sandy Bridge and Haswell being big leaps forward.
The Radeon 5000 series handled idle power consumption reasonably well, but not nearly as well as the 6000 series and later.
So, yes you could really reduce power consumption with newer parts. However, I'm with Torn, you're not going to beat the power consumption AND match GPU performance of your current setup for less than $100. The A8-5600K
does what you need, but is $100 for the CPU itself.