So i've got a boost guage, downpipe, and apex intake.
 

I'm going to change the vaccum lines and ast. I've flushed the radiater.

What mod should i do next?

Probably a new radiator :)

Miata thermoswitch or Temp Gauge Linearization.

Or boost and water temp gauges.

will a new radiator work with the stock IC?

Next Mod
 

After a downpipe and intake, I would consider either getting some sort of fuel management like a preprogrammed ECU which can be found fairly cheap used, a piggyback system like the Peter Farrell unit, or an Apexi PFC or similar. If you feel like living dangerously you can get a catback but with your current mods and a catback you may run into some lean conditions and we all know what that leads to. I dont disagree with the previous post though, a good rad upgrade is always helpful, even on a bone stock 7.

Rad upgrade
 

And yes, an upgraded rad will work with a stock IC.

If you haven't changed your tranny and diff oil you should do that too.

any recomendation on what oil to use?

75W90 Synthetic...I am currently running AmsOil, the same weight in both.

synthetic is ok for the tranny and diff, correct, just not for the motor?

Yes, it's ok....and if you start asking if synthetic is good for the motor you are going to bust out a huge can of worms on this forum...Some say good some say a bad...currnetly I run Penzoil 20W50 (non synthetic) as recommended by chris at RP.

Tranny/Diff oil
 

I am currently running Neo Synthetic 75W90 HD in the tranny. I haven't gotten around to switching out the diff yet, but what a difference in shifting. All the notchiness is gone. Neo is probably the best tranny fluid you can run, atleast from my experience.

What shifter are you using?

More upgrades
 

I dont know the current condition, mileage, etc. of your car but, if your looking to do maintenance oriented things, think about new plugs and wires, and changing out the fuel filter. Those and an oil change were done within a week of me getting my 7. Mine only had 35k on it but I figured I would start fresh.

Shifter
 

I am running the stock shifter. I have never been a big fan of short shifters. Driven a few WRX's with them and it made them feel notchy and hard to shift. Also drove a Civic with one and got the same feel.