Some simple CPU questions?

My 140 G3 currently has one E5130 @ 2.00GHz. I want to upgrade it;

questions are:

1. If I add second cpu, must it be same cpu - E5130 @ 2.00GHz ? Or I can make it a higher model from supported cpu list.

2. If I upgrade both at same time, can I use current heatsink for one of new cpus?

3. I found these pair cpus;

PAIR 2x Intel Xeon Dual Core 5160 3Ghz 1333MHZ SL9RT Woodcrest

and

PAIR 2x Intel Xeon Dual Core 5160 3Ghz 1333MHZ SLAG9

but can not figure which ones to choose. Supported cpu list says 5160 is supported. So which ones are correct for 140G3?

4. Do I need VRM for second cpu? I have some other hp servers. Would I need VRM for them with 2 cpus plugged in?

Thanks in advance.