Im building a new pc on a budget and my work horse is going to be The AMD AM2 5000+ Black edition because of its overclockability and Price/Power ratio. I've been eying up the Asus M2N-SLI Deluxe SLI mobo for $131, but sense this is a budget machine would I be able to get the same overclocking bang for my buck with say the Gigabyte GA-M57SLI-S4 which rides in at $80-90? The rest of the build mainly consists of a 9600gt card because of its power/price: which concerns me because its a PCIe 2.0 card, even though its backward compatible with v1.0 should I consider a 790fx or alike chip set because of the performance gain from PCIe 2.0?
I know you can't SLI on the 700 series chip set, but my budget is $500-600 so SLI isn't an option and you can still run a single Nvidia card on the 700 series chip set.
Thank you for your help

I don't think the motherboard should hold you back very much if you overclock using those unlocked multipliers. It should mostly be limited to the processor and your cooling solution. The motherboard would hold you back if you were bumping up the FSB.

You should check the list of supported processors. If you overclock and increase the voltage you might encounter some problems. Anandtech had some trouble with some AMD boards that were specified for 95watt processors only - they plugged in 125watt K10s and watched the sparks fly.