Whether or not you can under volt the CPU depends on the mainboard's bios (or UEFI) options.
You have to do a bit of research yourself, there are too many different boards to give a general answer. Xbitlabs
is a good source for reviews.
I would advise against a G620T, since it lacks certain features, most notably Quick Sync for video encoding (a major selling point for me).
I guess that's what I was wondering, if undervolting and setting the multiplier to a lower number was chipset specific or motherboard specific. It sounds like its motherboard specific which is nice.
I can get a i3-2100 for 99 at microcenter and the cheapest price on the i3-2100T i could find is around 130. Although the G620T does look interesting (83 dollars at newegg). I am probably going to be running a Radeon 6450, so I am not sure that things like 3D, quicksync, etc really matter to me. I have a i5-2500k running at 4.4ghz if I need to transcode something
The only other major difference is that that the i3 series has hyper threading.http://ark.intel.com/Compare.aspx?ids=53423,53422,53481