You will be lucky to hit 5 even on water.
Even harder with a cheap board.
+1 for this
Even with a high end board and great cooling, hitting 5.0GHz is challenging. 8GHz is only possible with sub-ambient cooling (IE: Liquid Nitrogen or a massive chill pack) and totally impractical for day-to-day use.
If you get a good chip that will OC on low voltage, you can probably get 4.6-4.8 on high end Air or a 120x240mm or 140x280mm radiator/CLC.
Some good budget OC boards:
Gigabyte GA-970A-UD3P
ASRock 970 Performance (Fatal1ty)
MSI 970 Gaming
All 3 of those have 8+2 phase power, good chipset/VRM cooling, and decent BIOS options for ocerclocking. They will only support 1 high end GPU though, so don't plan to SLI or Crossfire if sub-$100 is your budget for a motherboard.