Depends upon your notion of obsolete.
If asking if still under Apple support in 3 years than for the most part no.
http://support.apple.com/kb/HT1752
The discontinue date for the 2009 model is July 2010 and in three years from now it will be July 2015. It will have hit the 5 year mark.
Does that make the machine immediately unusable for work? No. It does mean you'll be scrounging for spare parts. You probably should not count on being able to run OS X 10.11 .
As long as you pull the Mac Pro off the Internet (e.g., set up firewall block between it and the internet. ) you can get use out it for an extended period of time even after Apple drops new security updates. You just won't be tracking supported versions of OS X and your apps. They will still work. If that is "fast enough" for your work then in that context it won't be "obsolete".
Pretty good chance that there will be a 2015 Mac Mini that outperforms you Mac Pro. In that sense, yes it will be obsolete.
The difference here is a relatively minor quibble for 2-3 year span for return on investment.
http://eshop.macsales.com/shop/memory/Mac-Pro-Memory#1066-memory
12 GB $103
24 GB $268
about $105. Over 3 years that's $35/yr. If adding the extra 12 GB can't generate a $3/mo improvement in productivity then probably don't need it. Also likely are going to have problems getting a newer Mac Pro later too.
Here is a simple way to say $105 on a new Mac Pro later. Just wait a couple months after the new model gets introduced and buy a refurb.
P.S. consider buying memory modules that will go into the 2010/2012 Hex core models. You may end up buying one of those in the future instead of a band new Mac Pro.