I could use some assistance with my new room......

[stinkyattic]

I'm very ambitious and I have some space

Good start!

I've got a 6' x 8' room for starters, and a kinda awkward 5' x 2' space stashed in behind. ......
I was kinda thinking of converting it to veg. room, and then turn the other one into a flowering room.

Yes, smaller space for veg, larger for flower= efficient

I have a 400w HPS and a 300w MH at the moment... Not ideal and I want to upgrade.....

It's a matter of adding a single light, and tossing the MH on a light mover run the long way in your closet. That will be GREAT for vegging. Then you can throw a couple more HPS in the large space. A couple 600HPS on air-cooling would be a good place to start.

There is a passive air intake. It sucks air in through the suspended ceiling in the basement. Air is vented outside through a 3" duct and bathroom fan. I'm convinced that it's totally insufficient,

Yup, it will be. You want to run your lights on air-cooled hoods and exhaust them all via a 6" duct with a carbon filter over the end, using a good quality blower.

All the walls are covered with corrugated PVC. I dig it. Unlike mylar it's non-conductive, and it's cheap like borscht.

Mmmm borscht. It may be cheap but it's delicious.

I'm unsure as to how I can turn this place into a hobby grower's wet dream....

Mattress (preferably stained) and pile of girlie mags in the corner?

I know i need new ventilation and lights. I don't really know what would be the best bet for both..... I have some sweet activated carbon air filters I wanna put to use, but I don't know what kinda fan would be best suited for my space......

Big fat 6" inline blower...mmmm ... as for lights, in that space, a total of 4 HPS 600s or 3 600s and your 400, would give excellent coverage.

Also I wanna get the max possible lighting for the size of my
room. I am going to throw some fluorescents into the 5' x 2' space. It should be ok for veging some little guys.

So that's it.... Let me know about any suggestions for pimping my shack. I want this place to be as efficient as possible, with as many constants controlled as possible. It's already discrete, just not as good as i could be...

You wanna pimp your shack? Light movers are yummy. Circular ones work for a nearly-square room, or a pair of rails on opposite schedules. A pair of rails with a pair of 1000HPS would be sweet.
Don't skimp on exhaust.
Shell out for a climate controller that will run your exhaust according to ambient temp and humidity, keeping your conditions controlled. Plan to keep a couple moms and use your veg room to care for them and for the cuttings they provide so you can run a very efficient watts : harvest weight ratio sea of green style grow.
And don't forget to have fun.