No networking connection after installing Win 2008 svr on a VM.

You need to select either VMXNET2 or VMXNET3 (if available and better).

and you will need to install VMware Tools for  VMXNET2 or VMXNET3 drivers and support.

see my EE Article

Part 8: HOW TO: Install VMware Tools for Windows on a VMware Windows virtual machine on a VMware vSphere Hypervisor 5.1 (ESXi 5.1) Host Server

Then how to change the nic from vmxnet2 (enhanced) to others at this point since in 'Edit Setting' the nic selection for this vm is dimmed out.

So I need to install VMware Tools for every VM, correct?
After I installed the Tools on this VM, I seems to get the network connection now. Do you think I still need to change the nic from vmxnet 2 (enhanced) to others? (I'm asking since I'm afraid some issues may be hidden for this nic on this version of server.)

Yes! You need to install VMware Tools for every VM, you may need to read my EE Article then!

What version of ESXi, if 5.x, then VMXNET3 is better.

mine is ESXi 6.0. Do you think  I still need to change the nic from vmxnet 2 (enhanced) to others?

Yes, you should really be using VMXNET3 for every VM.

Okay. got it.
A side-tracked question... If a vm is currently power off, it will not currently use its assigned memory or all other resources, correct?

Correct. It will use storage space though!

Thanks Andrew. Another question, if we need to change the resources, say increasing memory, do we need to shut down the vm?

All these should be new questions, which you should post. So last one!

Yes and No, depends if the OS supports Hot Add Memory, and the VM is configured for Hot Add.

But, usually yes.

install the VMware Tools first then add one nic xmxnet 3, and then remove the current nic