The prices on some of this stuff should drop real soon now, making this a doable alteration. However, good luck on whatever you do. Intel and AMD will both introduce 600Mhz processors and the prices for the rest of their CPUs will drop through the floor. There will even be a new 500Mhz Intel Celeron, too, so Celeron prices will also drop. Now, since you feel like using your monitor as a television and your computer as a DVD player, this changes some the priorities of where you need to spend your money.
SCSI burners require SCSI controllers.
Current DVD players are not multi-threaded, so having multiple CPUs won't help. However, Intel has added new x86 instructions to their Pentium III CPUs that will help with DVD, MPEG and MP3 playback. AMD has a similar set of instructions. When more money comes along, you can later upgrade to the PIII 600's that are just about to come out. Buy a GOOD case or you'll regret it.