backwards compatibility

I'm wanting to upgrade my system, and it seemed to me that I could make a much nicer set up if I reused some of my old components (hard drive, ram, dvd rom) and only had to spend money on upgrading to core stuff (mb, gpu, cpu, psu). However, the motherboard I'm looking at supports ddr2 800 speed memory and what I have now is 667 speed. Also, my current hard drive is ultra ATA 133 and the new motherboard supports ATA100. Does this skrew up everything or will these older components still run in the new system??

 

I'd certainly keep your old hard drive, but get some new ddr2 (Corsair XMS2 is my personal preference).