The TigerMP is generally the least expensive board to be found. New ones can be had off Pricewatch for about $168.
Considering how finicky these boards are with Radeon 8500's/Geforce 3's and 4's/some brands of memory, I'd avoid them unless you can buy the stuff that's known to work with it. I'd suggest to ante up to an MPX board of some sort.
For what it's worth, you can use up to two sticks of non-Registered memory in MPX motherboards. Most people suggest going with Registered, but if you already have a stick or two of regular DDR, you can use them.
If you want to use a third or fourth stick of RAM, though, they all need to be Registered.
A refurbished Tyan or Asus dual, will be the cheapest way to get a dual. A motherboard is an investment, though, so I suggest getting a really good one. It will be money well spent. The favorite dual motherboard around here is the MSI K7D Master (I have three of these and I love 'em.) Quite a few people, especially at the 2CPU forum, have had very good experience with the Iwill MPX2 (it allow you to set the FSB over 150 MHz, which the K7D does not. The MPX2 does not have multiplier control, though, which the K7D does have.)
Those are the only motherboards I can suggest, as they have the best overclocking potential.
Stalka, yeah, you can run dual Durons, if they're Morgans. Most dual AMD boards wil run them, although, you may have to change BIOS' to do it. (For example, on the MSI K7D Master, you have to use the old BIOS 1.0 (or maybe it's 1.1) to run Durons. None of the newer BIOS' support Durons, for some reason)
The CHEAPEST possible way to put together a dual, would be to get a refurbished board from Newegg (~$100) and get a pair of Morgan Durons.
Gnerma, PM theotherphil. He has an Iwill MPX2, so he can tell you for certain. I used to have one, but it was junk, so I sent it back. If memory serves, it does have Vcore and the FSB goes up to 200 or 250, though it only has a 1/4 PCI divider. Personally, I don't recommend the Iwill, but I don't want to get flamed by not mentioning it as an alternative