What power range should I move into from the 300?
At least 400w, but make sure it's an Enermax, Antec, or Vantec.
Any specific recommendations for power supply?
See above.
Is replacing a power supply fairly easy?
Yes. Retardedly easy.
What neon light is recommended (small, cold cathode, dual lights, one, two, three colors,red,blue)?
Stay away from neon because they get hot. Go to www.coolerguys.com and get their dual-cold cathode sets. They're on sale this week!
Side panel or Etched Appliqué?
Whatever you want.
Any other custom case recommendations?
Get a Lian-Li. For inspiration, go here and check mine out. Also, search this forum for sixate's case because it's pretty dope, too.