Homemade PC EKG circuit
I that think you need to add a driven right leg circuit at the first for patient safety and  protection to bypass the excessive current in case of contact with live wire.
Also for proper reproduction of waveforms on PC make sure to isolate your analog ground and your controller ground.

Also for Ecg, gain of around 10000 is enough.You can split this gain between two stages to avoid saturation and add a 60hz notch filter between the two.

Eg. stage1 amplifier(Instrumentation amp)----notch filter------stage2 amplifier.