A good overclock-able CPU is money well spent IMO.
I kept my Q6600 for ~7 years pushing it harder and harder till it eventually held my gfx back too much.
Upgrading gfx cards is easy. CPU, not so much. Almost always involves a new board, sometimes ram, and a lot of buggering around.
If you can wait and spring for a 6600K, it will last for many years.
I'm expecting my 4790k to also last ~7 years.
If you've got no choice, the 6100 should see you through the next 3 years quite well, but maybe not more than that if you're upgrading cards along the way.