Need some ideas for upgrades - 07 600 RMK

Depends.
Need more info
What are you going to do with the sled?
Deep powder, trail, lake race, ditch bang...........
how much experience ya got?
How old are ya? (don't want to tell ya to put nos on it if your 14)
How much ya looking to spend??
How wild ya wanna get?

Me.....................
carls 660/685 kit (including v-force 3, pipe and can)
Holtz spindles.
Gearing depends on track length and what you want to do with it. Usually I drop the top gear one tooth.
That right there would pretty much use up 3500-4k.

Gearing that I don't know. I ride 8k+. I have never even ridden below 6k.
For under a grand, I would do the Holtz front spindles. 300 dollars. Really improve the handling.

I put the holtz footwell vents on my 08 700 to direct the air flow better. You can also do the holtz shock tower vents. That combination will run you around 150.

Slp intake (replace the little foam inserts). You can also put a couple 2" intakes to increase air flow intake and do the air horn. That setup will run you around another 100-120.

If want better snow evacuation from your running boards (no ice build up) get a set of Better Boards from F-bomb. He has links to his web site. Just do a search for better boards. That will cost you between 150-200 (depending on if you want them powder coated or not). Those will be going on my sled next.

the SLP pipe fits nicely into your budget and really wakes up the 600, if you get one for a stock silencer you can still use your hps can. gut your airbox and put some intake vents in it. Let that 600 breathe and you will be impressed!