If you want to get a Pave low or AC-130, my advice is to change your lower kill streaks.
Because kill streaks add up in MW2, for example if you have a predator missile, and kill two people with it, you can get a 7 kill streak.
So what I do is have predator missile(5), attack helicopter(7), pave low(12)
Basically I just aim for 5 kills. Get your predator, and when I get my attack helicopter I tend to camp. Although I hate camping, it's just a lying in the bushes camping while my chopper shoots people until I get my pave low. Sometimes either, someone shoots the chopper down, people are hiding or have blind eye on, so the tough part is getting to 12 without dying if that happens.
It takes skill and knowledge of the game. Just get practicing, check your corner, don't run out into huge open areas, it's pretty straight forward, just have to use common sense.
For guns, on a larger map like Bakaara, Resistance, Downturn, Lockdown, Village, Fallen etc. I tend to use a fully automatic assault rifle. So if i'm having a front on confrontation with an enemy, I have a gun that I am comfortable with and that I also know that kick someones ***.
In small maps I bounce from assault rifles, to sub machine guns, to shotguns. You just wanna find a gun that you feel fits right for the map.
For perks, it really depends on what gamemode you play. For example, I play hardcore TDM, and hardcore KC.
My perks are
1. Blind eye/Sleight of hand - I use blind eye if they have airsupport, otherwise, sleight of hand saves my *** a lot.
2. Hardline/Assassin, I tend to like having assassin on just so I'm not noticed. But because on hardcore you don't always have a UAV up, I sometimes switch to hardline, so its easier to get my kill streaks.
3. Marksman/Stalker/Sitrep/Steadyaim now I really go through these perks. I use marksman when i'm in a long distance map, so it's easier to see peoples names. I use stalker when I have a sub machine gun so it's easier to move around when i'm aiming down sights around corners, sitrep, when we're in a map with a lot of building (lockdown), but if you have stalker pro, you don't need to worry about that because you delay equipment when you run past them. I use steady aim sometimes when I use a shotgun (striker) because I tend not to aim down my sights.
I hope that helps and good luck being a badass on the game! :)