Im not 100% sure but i think you might be trying to put DDR2 ram into a DDR1 motherboard which will not work. Tell me your processor (Athlon XP, Athlon 64, Pentium 4, Pentium 4 LGA 775) by downloading CPUZ if you dont know so i can work out what to do.
Is the motherboard DDR2 or the Ram DDR2? Im unsure if DDR ever came in 400mhz varieties. Anyway, still download CPUZ and get some more info to me.
Core 2 Duo E6400, DFI Infinity 975X/G, 2x 512mb DDR2 667mhz, Albatron 7900gt, WD 200gb SATA, Samsung DVD-RW, Silverstone ST-50EF 500w PSU.