Athlon II x4 are very good for their price. 620's can be had for $85 on Newegg and can match a Q8300 if oc'd. 955 BE is roughly equivalent to a Q9550, can be had for $139.99 on Newegg w/ free shipping. If you can stretch the budget get the 955, otherwise the 620/630 are very good entry quad cpu's. The 620/630 will save you about $100 which you can put into a better gpu/etc.
Review of 620/630
http://www.anandtech.com/show/2836/9
Review of 955 BE
http://www.tomshardware.com/reviews/phenom-x4-955,2278-12.html