Upgrading to Win7 64 and 16GB RAM - what to do first?

My office workstation (architecture office running graphic-intensive 3D software) runs dual dual-core Xeon processors with 4GB RAM, an 80GB 10K RPM HDD, and XP Pro 32-bit. This machine is almost 3 years old, so I'm doing a bit of upgrading.

The HDD has been making more noise lately, so I'm replacing that with a 640GB 7200 RPM 32MB cache HDD, and re-loading everything. I'm also going to switch to Windows 7 64-bit. The mobo has 8 memory slots, so I'm installing 8x 2GB sticks to replace the current 4x 1GB sticks (mobo can handle 32GB, 8x 4GB sticks maximum).

My question is, what should I install first? Should I install the hard drive and OS first, while still on the 4GB, and then install the 16GB after the OS is up and running? Or should I just go ahead and install everything at once? Does it matter?

Install 16 GB RAM first, even though my current OS is 32-bit?

There is no difference. Do it the way you prefer.

Install the RAM and run Memtest86+ from a bootable CD. I had major headaches installing Win7 and only later did I realize I had bad RAM. Had I known that up front, I would have saved myself a lot of grief.

Your motherboard obviously likes the 4GB of Ram that is installed right now.
What I would do is swap out the HDD and install Windows7 with the old ram.
When all is up and running start installing your New Ram. If you install it all at once , ( 8 sticks ) and you have a problem, it is going to take alot of time to figure out what stick has an issue or what motherboard slot. Again, what I would do is install the new Ram in pairs with testing between. This way if a problem comes up you have already narowed it down to 2 sticks and 2 slots. Also when you have all 8 sticks installed you will more that likely have to increase the DDR voltage a little to handle the additional PCB's/Modules.

Edit: The recomended voltage for your RAM should be printed on each module. That is the minimum voltage, either varify or set in Bios.

If you put all 8 sticks in and you dont have any issues ( like system freeze or BSOD ) then you are good to go.

Well I got the HDD and OS up and running today, but I got the wrong memory (unbuffered DIMM's instead of FB DIMM's). So I will have to send these back and get the correct FB DIMM's to install later on.

What I didn't realize is that the Win7 upgrade version wouldn't work for a fresh install on an empty disk, and it wouldn't tell me that until AFTER it had already copied all the files and asked for my product key. So I had to quit, install XP, then install Win7 again to get it to work.

I was in a similar situation - have a copy of Vista but didn't want to install it just to overlay the Windows 7 upgrade. This is a moot point for you now, but if you ever re-install or for others reading this thread I found this to be helpful:
http://www.winsupersite.com/win7/c [...] _media.asp

I was able to successfully install on a new drive following the author's "Method #1".