Penryn support means you need something based on P35 or X38. Crossfire support is best in X38 where both cards get to run at x16. With most P35 boards you end up with x16+x4 and that's not so good. Also, avoid DDR3 motherboards.
Any motherboard based on the X38 chipset, and any P35 board with 2xPCI-E x16 slots will run CrossFire. As for recommendations, Asus P5E, Asus P5K-Deluxe, GigaByte P35-DQ6 or Abit IP35-Pro.
The X38 chipset based boards will have 2 full speed PCI-E 2.0 X16 slots, where as the P35 boards will have one X16 and one X4 PCI-E 1.1 slots. As far as what difference that will make in crossfire performance, depends on the game and the settings, and this was tested with HD 2900XTs. In some situations it made a decent difference, in others, it had no affect on performance.
Bit-Tech's test here: http://www.bit-tech.net/hardware/2007/10/12/crossfire_c...