Hi,
I'm new to mining. Can you please help me setup mining rig?

I've bought HP DX 2400MT with some Core 2 DUO CPU... When it arrive I will check it, I want 45nm CPU for better cooling.
https://support.hp.com/us-en/product/HP-Compaq-dx2400-Microtower-PC/3658372

• CPU: 45nm Core 2 DUO or maybe QUAD
• RAM: It has 4GB, do I need more?
• HDD: 320GB
• GPU: I'm trying to buy HD 7850, R9 270x and cheap 7770 for start.
  My plan is to change them after, to at least 3x 7850 or better.
• PSU: It has 300w, but I want buy new Zalman ZM700-LX
  Is this enough for 3x 7850 or 3x R9 270x?
• Cooling: I have some 12cm Arctic fans.
  Also I don't know what is CPU cooler, so I will buy some old heatsink if it's not good.
  I'm trying to buy some OC cards with better cooling than reference, I will only put some Arctic MX2 on it, maybe MX4 for the more powerful ones.
• OS: It has Win 7 64bit, is it good or update to Win 10?
• Mining SW: Claymore? I'm using now XMR Stak NVIDIA there is also AMD version, will try anyway.

Will the PSU be enough? Any advices for me?
Is USB 3.0 power riser good or should I use ribbon?
I'm mining with 860m on my laptop now on Minergate XMR+FCN and also with this rig I want too, any advices?

Thank you for any help