Abit NF-M2 nView (if available) gets highest clocks (350-400MHz), costs close to $100
Biostar Tforce6100-AM2 doesn't clock as high (300-340MHz), 2.0v vDIMM only, around $75
Biostar Tforce7050-AM2 newer version of above
Biostar Tforce690G AMD chipset, lower clocks (265-310MHz)
I've also heard that EPoX mATX boards are nicely overclockable, but they're pretty much gone out of business these days.
My choice for cheap and overclockable would be the Biostar boards, maybe the 7050. Hopefully it has higher vDIMM than the older board. For the most part being able to hit 300MHz HTT is "good enough" because though the Abit can go much higher, most CPUs have high enough multipliers. Even the Brisbane 3600+ would be at 2.85GHz at 300MHz HTT, which isn't shabby at all.