I would like to put a new track on my sled, 2004 xcsp 500, and at tracksusa.com you can get a 40$ discount when you buy a track and extension kit
What would be a good track preferably studded, and would it be worth it to get the extensions?
And if I were to get the extensions what other parts would I need?
Also, What is the pitch for this sled?

What track to get would depend on your riding style/conditions. It uses the regular 2.52" pitch.

75% of my riding is on the trails but the rest is just some deep powder

Prepunched 136 X 1 1/4" ripsaw is what I would tell you to get. It has a 2 year warrenty with studs , no othere track is warrenty unless its prepunched by camoplas. 3 seasons and counting on my ripsaw thats perpunched zero tearouts , not from the lack of trying I must have replaced 20 broken studs so far ! The Ripsaw isnt the best deep snow track but it works ok , it works great on hard pack and you dont loose much top end with the taller lugs. I only run 108 studs in mine , try it like that first you can always add some later.

Does having the longer track give me more "grip" or less darting, and stuff like that because that was what i was hearing?
With the longer track and more studs/track on the ground at once, what would be a good carbide length? I would they rather be single, just because i like how they turn
And what about the bergstrom tripples? What is everyone's view on those?

the ripsaw will give you WAY more grip than your stock track could ever dream of. Probably as good as a stocker with 96 studs. Longer will give you more traction. Also it will grip better in the corners and cause the front to push more than before. I went to dual 6" on my 700 after installing the ripsaw 1.25 and it took care of all the darting and it turns like it's on rails compared to before. Like xc-mark said, not really great in deep snow, but better than stock anyway. Awesome on groomed and packed trails. If you are set on studs, get the one that is pre-punched like he said too, but I would recommend trying it without the studs first unless you run on a lot of ice. I've just seen to many studs in heat exchangers ruin a trip, but the pre-punched seems to hold them better.

Yeah my dad put dual 6 inchers on his and really likes it, no darting
I run on a decent amount of ice, like enough to get studs but how many for a 136"? I have 144 with backers on my 121" now, with 4 inch singles on the front, broke down and went to four stores before we found anyone with carbides, bought them without thinking about getting better ones i ended up paying 56 fricking dollars on 4 inch singles
And the funny thing is i run studs on the outside part of the track 2 studs every other lug, and then two close in one lug, and two farther away the other lug, everyother lug and i have had no pull throughs or broken studs, just a couple of the plastic backers missing

save your singles to sell next fall when someone needs them or to your buddies. you may take a hit on them, but the duals are well worth it. But, they are 56 each instead of a pair. If you get the pre-punched, it will have the maximum recomended number of holes in it. I do not run studs, but when I did, 96 was usually good on a 121, not sure on a 136. I'm sure someone will chime in and have a recommendation. Check Tacksusa.com for good prices on extensions and track combos.

108 is 2 on every lug on a 136" track. It has 54 pitches. Check around with your local dealers sometimes they have take off tracks for cheap. I picked up a 136" ripsaw on Ebay for under $400. Rail extensions also on Ebay.

I am most likely to get the stuff i need on tracksusa.com, except for the track i run a 2 to 4 stud patter per lug on mine, and like traction i get

I don't know where you live, but you might want to check out an event called Haydays. Its a huge snowmobile swap meet just outside of Minneapolis, Minnesota that goes on every year around the first weekend in September... you can find everything there both new and used for MUCH MUCH MUCH cheaper than anything online. If you don't mind a track with a little use you can save a ton of money too. I picked up a 121 X 15 X 1.5 last year with a couple lugs missing for $50. Just a suggestion, check it out.