Server 2008 R2 boot is at 2 hours and counting. What now?

Solution 1:

After 2 hours trying to boot, I don't think you have a choice. If it hasn't booted by now, it's not going to. Time to power it off, hard, if needed.

And FWIW, those timeouts can be caused by bad or incompatible RAM. Pull out the new RAM before booting the system up again, or boot into a diagnostics/MEMtest CD and see what's up with those new DIMMs.

Solution 2:

Sounds like bad RAM. (I see you had the presence of mind to do a reboot test on the machine before you started mucking about with hardware updates...)

First step: Turn the machine off.
If you can, try a remote shutdown, but it sounds like you're at a point where the machine is totally unusable in its current state, so if it takes more than say 15-30 minutes to shut down go ahead and have it powered off.
You might do some damage, but honestly you may be doing more by letting it churn away with what is probably bad RAM. You don't know what's happening behind the scenes at this point, and a hard power-off is a relatively known quantity.

Second step: Undo the last thing you did before the machine started misbehaving (pull the RAM).
If that makes the problem go away you probably have bad RAM. If not you have some other problem...

Third step: Reinstall the RAM. Make sure it's properly seated.
Run a memory test program (or this one, or many others) before booting the OS. As David said in his comment you should really do at least one pass through one of these tools whenever you install RAM. Bad chips are not as common as they used to be, but they do still happen.