The Phenom II X4 840 runs at 3.2GHZ and the Athlon II X4 645 at 3.1GHZ so if you can get a Phenom II X4 840 for around £80 it is worth the price.
However,the 3.2GHZ Athlon II X3 450 can be had for around £58 and is basically a Phenom II X4 840 with one less core.
http://www.overclockers.co.uk/showproduct.php?prodid=CP-280-AM&groupid=701&catid=6&subcat=1328
Is this the one your speaking about?