How identify Power Egde 2950 Motherboard

You will see a label on the mother board something like CN-07H243........  7H243 is the actual Dell part number.  Post any numbers on any labels you find, and I'll tell you what the part number is and likely what generation it is, although this last part can be more difficult by part number alone.

Well, it will be necessary shutdown my server for that, however, that is not a problem, the situation is that I want to buy a new one, and how could I check the new board or make sure placing the order?

Ok, that wasn't clear from your original post.  Assuming you already know what version you have and just didn't post it ...

Best/easiest way?  Call Dell Technical Support - 8004563355 - free whether you are in or out of warranty.  They have a readily-available list of part numbers for the 2950, which shows which generation of board it is.

You can also do it the old fashioned way ...

http://accessories.us.dell.com/sna/productdetail.aspx?c=us&l=en&s=bsd&cs=04&sku=CU542&mfgpid=167754&chassisid=8726&~ck=dellSearch&baynote_bnrank=0&baynote_irrank=1
http://compare.ebay.com/like/170558884477?ltyp=AllFixedPriceItemTypes&var=sbar&rvr_id=171551249485&crlp=1_263602_304662&UA=%3F*I8&GUID=c234207a1270a02652e4a6b4ffca45f3&itemid=170558884477&ff4=263602_304662

If the front of your system chassis is labeled with a “III”, your system is upgradeable to the 5100 and 5200 series of dual-core Intel Xeon processors and 5300 and 5400 series of quad-core Intel Xeon processors.
http://support.dell.com/support/edocs/systems/pe2950/multlang/ts/IU2950.pdf

The letters: DT021 can indicate some specific about the motherboard?

That is a Dell part number for a PE 2950 II motherboard.

If the front of your system chassis is labeled with a “II”, your system is upgradeable to the 5100 series of dual-core Intel Xeon processors and the 5300 series of quad-core Xeon processors.

Exactly, but, there is not such label on the front; so, bottom line, seems like I can install two E5345 CPUI's in this motherboard with not problems, right?

So it would seem.  The II label is often just a sticker, so it may have been removed or fallen off.  To confirm that ... if you go here and enter your Service Tag and change the tab to Original Configuration, you should see it in the details some where as something like "PE2950, II" - probably under the motherboard/planar or assembly/chassis:
http://support.dell.com/support/topics/global.aspx/support/my_systems_info/details?c=us&cs=08W&l=en&s=pub&~ck=anavml

You can also double-check it in OpenManage Server Administrator, under System, Properties tab, Summary section ... see attached.
2950iii.jpg

UJ032 ASSEMBLY..., CHASSIS..., 3.5HD, 2U, PE2950, V2  ????