MacPro from 2.10GHZ/12M/5.86 -to- 3.20GHZ/8M/4.80

NO, RAM is not the problem.

your Mac has error-correcting code RAM. Wrong RAM will cause blinking power light and failure to start.

What model Mac Pro silver tower? what version of Boot ROM?

The initial "chime" sound is generated in software when your Mac passes the Power-On Self Test. If the chime occurs and/or startup continues, your Mac is working. The blank gray screen should light up. Then on to the disk Drive.

Accessing the Boot drive:

The solid Apple is not in the Mac's ROM at Cold start. The Apple logo can only appear when it is fetched in the first "blob" of software loaded from a 'magic' place on the boot drive, or re-run after a Restart. Then a whole lot of stuff is initialized, and the progress Bar moves part way across. After a cold start, seeing the solid Apple appear says your drive is not completely dead.

Mounting the Boot drive:

The next step requires a lot of files by name, so the File System is initialized, and the Boot Drive is Mounted. If the drive directory is damaged, the drive can not be Mounted, so your Mac begins one pass of Disk Utility Repair. This will take an additional about five minutes. During this process, the progress bar may be extended, and will grow by an additional amount not seen on a routine startup.

at the end of that process (which should not take more than about five minutes), it will attempt to Mount the drive again:

-- if the drive Mounts, boot-up continues.

-- if the drive cannot be Mounted, your Mac can do nothing more, so it powers off.

-- if the process stalls, this may indicate you have Bad Blocks on your Rotating Magnetic Boot drive (if so equipped). The re-reading of Bad blocks can take a very long time (on the order of a quarter minute for each Bad Block).

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If you got the solid Apple on the display, your computer is WORKING. It has already executed millions of instructions and done some Disk I/O successfully.

You just have a few details about the rest of your system to work out.

Most folks would not have given up two six core processors for two fast four core processors, but to each their own. Your computer is working.

Well, that one maintains 12 processors, so most would take that over 8 processors. The more cores, the slower they tend to get, unless you are willing to pay a lot more.

It looks pretty nice, but as always, it depends on what it costs.

Yes. The slower RAM will run, but at its rated slower speed.