GS don't use GPU at all

[Nexxxus]

The 2500k is decent enough for GT3 and 4, at least when MTVU is turned on.
Your problem is obviously the GPU. A 8800 GTS is old and is totally bottlenecking your CPU.
Get something decent like a GTX 950 and you're good.

Even if the GS percentage doesn't go higher than 50%, the GPU or the vram itself might be on 100% load,  you can check that using HWMonitor or MSI Afterburner.

[Topken]

I personally use this for monitoring the GPU

https://www.techpowerup.com/gpuz/

[Nexxxus]

that works too.
But I'd recommend getting a better graphics card anyway since a 8800GTS bottlenecks the CPU.