I don't know if your dead set on buying a amd CPU but maybe you should look into the new intel core 2 duo's, currently there leading the performance race, with compitive price's with amd. For a cpu i recremend a E6400 best cpu for overclocking for it's price you can get most mhz out of that but since your new to building computer's im not sure if you would be into overclocking.
Intel Core 2 Duo E6400 Conroe 2.13GHz 2M shared L2 Cache LGA 775
http://www.newegg.com/Product/Product.asp?Item=N82E16819115004
If your not into overclocking or your not too sure how to do it the one 'chupacabra' recremended is great.I've Used an AMD athlon 64 4800+(overclocked to 2.8ghz) for the last 5 months and that was great and i only recently changed to intel,(core 2 duo E6400 over clocked to 3.2ghz)
These are your basic motives for buying motherboard and cpu i guess.
Spending more on a motherboard, means better for the future, usually means more features, and lets say. overclock's better. and generally last better.
Spending more on a CPU u get more performance basicly it on that behalf(but a major one)
I'd say find a motherboard
-that supports the latest socket(either LGA775(make sure it supports atleast core 2 duo's if it supports quad's then even better)(intel) or AM2(i think these all support duel cores)(amd))
-that supports PCI-express(jst 1 will do, 2 usually cost more on a mobo and pluss with sli or crossfire(duel graphics card's) they cost quite alot and for the price of 2 you can get 1 card that's better than the 2 put together)
-that supports sata with raid 0,1,3,5 would be great. alota low end to mid end mother boards support least some kinda raid.(if it doesnt have sata then you dont want it)
i would say those are the main things, but here are some extra's that would be nice, so if you find a board that has those 3 things that's in ur price range then start looking for these things
-one that supports duel channel ram
-8 channel audio system
-gigabyte lan
-duel gigabyte lan
-wi-fi
-sclient cooling
-firewire
-sata 3.0GB/s
but remember spend wat you need on a motherboard and spend the rest on a good cpu i'd say for performance
(man long post sry guys)