Run a memory diagnostic like Memtest86+ (do a Google for it, then burn it onto a floppy/CD-RW) to check for bad RAM. If you're having problems with only one DIMM then it's likely a fault with either the other DIMM or a BIOS/Chipset bug.
Is the mainboard's BIOS up to date? In BIOS, is the CPU recognised?
Upon the installation of the new motherboard, did you fomat the HDD correctly?
AMD X2 4400+ Toledo 1MB/Asus A8N-SLI Deluxe/2GB Kingston HyperX/SLI Gigabyte 7800 GTX 256MB/Creative SB Audigy 2 ZS Plat w/ Creative Inspire 6.1 speakers /180GB Hitachi PATA/19\" AG Neovo F-419 LCD/OCZ PowerStream 600W PSU/WinXP Home SP2