my understanding is the N450 with built in graphics plays MOST dvd fine. It is especialy important to increase the systems memory to 2gb minimum (many support that as maximum unfortunately)
newer netbooks with ION 2 or Broadcom Crystal HD chips make the load much less on the cpu and I woould think you would want one of those if DVD playback is what you plan on alot versus copying movies into the hard drive and playing from there.
if seemly playback is a paramount issue i would recommend you get the biggest fastest SSD drive, and one of the afore mentions graphics chip netbooks and copy movies onto and play from the ssd and you wont drop a frame.
The performance for this external optical drive you mentioned, will not be based on the processor only but on the whole system configuration (memory, chipset, BIOS, etc). We recommend contacting the OEM (Original Equipment Manufacturers) for further information and assistance.