In need of CPU/Mobo/RAM upgrade advice, ~$500 budget

I've built PCs before, but this is the first time I'm selecting parts myself instead of going with free salvaged stuff, and now that I'm looking to do an upgrade (go go tax return money), I'm not sure what works best for my needs. My use-case:

• this is going to be a combo media/server desktop
• dual-boot Ubuntu/Win7
• I'd like to be able to play Blu-ray quality video without lagging/artifacting (but I don't need 4k)
• light video editing/3D CAD work
• some gaming, but I tend to be 5-6 years behind the times and not super-interested in hyper-realistic games (Dragon Age and Mass Effect are the newest series I'm interested in playing, and I'm still way back at the beginning)
• compulsive multitasker
• will probably be running VMs fairly frequently

I'd like to be pretty solid for the next four or five years. All the other parts I've got sorted, and I'm looking for mATX or ATX form factor as that fits the case I've got already. Budget has a little bit of flex in it if the gains are significant.

The current parts-list for the machine: PCPartPicker part list / Price breakdown by merchant

(the mobo/CPU in the machine are +10 years old and dying, so they're junk at this point)

I know the GPU is crappy, but it's functional where the mobo isn't any more, so it'll be a seperate upgrade later when I have the spare cash.