I had 230 watts of PC lighting (combo of 2 8,800K and 2 Ultra Actinic bulbs) on my 65 flatback hex, and just added two 175 watt 5,500K MH bulbs. The difference is incredible! I am very happy.
I got DIY MH retro kits from <a href="http://www.aquaticlight.com" target="_blank">http://www.aquaticlight.com</a> for $120 each delivered, with the upgraded 5,500K bulb. No reflector or wiring harness, but I was mounting into a custom canopy so it didn't matter.
As I understand Kelvins (and I'm no expert) the higher number has to do with color (and presumably broader spectrum) so if you already have PCs, you probaly don't need to get the 12,000K bulb upgrade. Somebody who has MH + VHO/PC lights and has tried different MH bulbs might give you a better answer on how it looks.