I've decided on DDR3 for future compatibility. I don't upgrade my PC very often (I'm still running an Athlon XP 2600+ on an Asus A7V333) and want to try to minimize any components that I won't be able to use the next time around. I already have to buy literally EVERYTHING (motherboard, CPU, SATA drives, memory, PCI-e video, better power supply) for this upgrade. Only the case is being reused. I'd like to avoid that next time.
I know that the main difference between X38 and X48 is official 1600FSB support. I've also read that most current x38 boards can "overclock" the chipset to support 1600FSB CPU's, but I'd think that it might cause problems with the already hot X38. I'm guessing that the X48 will run cooler than the X38 but I have no proof of that.