I own the same motherboard... the M3A78-EM is a fantastic little motherboard that supports everything to date ! Ignore the above comment it's a waste of money to go AM3 at the moment since you already have the ram.
I currently run a Phenom 965 BE c3
It's a quad core @ 3.4Ghz with 512KB l2 cash per core + 6MB L3 Cash...
All in all, it's a good chip... doesn't hold a candle against sandy bridge I5 but it does a great job for the price and your best upgrade path.
You can also pickup a phenom 965 for cheaper. it's a 3.2Ghz and you can overclock it to 3.4 or 3.5Ghz very easily...
The M3A78EM also supports the thuban 6 core processors but I do not recommend it.
You will have to go to the bios, and verify the bios version of your motherboard, you might need to update it... but I am pretty sure you are already good for a Phenom X4
Check this link
http://support.asus.com/cpusupport/list.aspx?SLanguage=en&p=1&s=24&m=M3A78-EM&os=&hashedid=KjpYqzmAd9vsTM2D
140$ free shipping... 3.2Ghz Quad, this destroys your old 6000+
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103808&cm_re=955-_-19-103-808-_-Product
79$ Free shipping... 3.2 Ghz Tri-core slower then the phenom, but also destroys the old 6000+
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103886&cm_re=Athlon_X3-_-19-103-886-_-Product
136 + shipping... 3.0Ghz quad (since you said you only wanted 3ghz...)
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103809&Tpk=phenom%20945
Phenom 965 or 970 isnt really worth the extra premium, Phenom X6 might do you good if you use heavily threaded programs for animation... but that involves flashing your bios... it's quite easy... download bios from asus site, put it in usb key, go to bios and flash away... Well, Read up on it and make sure you know what you are doing while doing it... in dough... don't do it.
Athlon X3's and X4's are great little chips for the price but without the 6mb l3 cash that the phenom has... Phenoms are about 10 to 20% better in games clock for clock against an athlon.
Hunt around the site for the benchmarks.