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.