The ASRock 939Dual-SATA2 is a great value oriented board for AMD Socket939. It has both a native PCI-E x16 and an AGP 8x slot.
Unfortunately the BIOS is lacking overclocking options.
With an AMD Venice CPU the maximum CPU voltage available is 1.450V, definitely not enough for some serious overclocking.
Settings for memory voltage are even more limited: "Standard" (2.6V) and "High" (2.7V) just doesn't cut it.
Another issue is that with all official BIOSes you will have great trouble overclocking beyond 275 MHz HTT (v1.40 was the latest when this article was written).
OCWorkbench has released two tweaked BIOS files
which fix the 275 MHz clock issues.