why go with the older dated Am2+, for few bucks more:
ASUS M4A79XTD EVO AM3 AMD 790X ATX AMD Motherboard - Retail /AMD Phenom II X4 955 Black Edition Deneb 3.2GHz Socket AM3 125W Quad-Core Processor Model HDZ955FBGIBOX - Retail , nice arse board, can get some DDR3 you coould use later (unless of course you're re-using some DDR2 ne.), plus with the BE, you can tweak out with the unolcked hammer.
There's other combos with the CPU for less.
May be a stretch, but you're not far away from a Intel Core i5-750 Lynnfield 2.66GHz LGA 1156 95W Quad-Core Processor Model BX80605I5750 . Budget mobo, and a kind mild Oc, 4-4.2ish, then you;d be doing some gaming.
Food for thought, realize its a it more, but it'll last yah a while.
But b/w the 2 you've chosen, I'd take the 8400, b/c it'll OC like a champ amd game great. Got an E8500 kickin at 3.8 gs and it games stabe sweet.
Penewah!
Box 1:Xeon W3520 D0/HeatKiller 3.0/Feser Extreme 360 w/ 3x Gentle Typhoons 1850s/SWFT655/EVGA E760 Classified/ XFX GTX 285(Kool 285 block OTW)/CORSAIR CMPSU-850HX/ CM HAF /Corsair Domi's 3X2GB DDR3_2000/F1 Spinpoint 2x1TB-SATA/HT Omega Striker/ Build #2: Antec 1200/ E8500 @ 3.87 Ghz/Gigabyte P45-UD3P/ OCZ 4Gb Reapers PC2 9200/EAH4870 1GB/750W PCPC Quad Silncer/CM V10 1/2aTEC/ #3 Biostar 790GX/AMD 9950BE 3.2Ghz/OCZ Reapers 4GB 1066 /2.75Tbs of F1 Spinpoint/Corsair 550W PSU/