Close-Quarters Set:
UMP45 with Holographic Scope and FMJ
Bling Pro
Stopping Power Pro
Steady Aim Pro/Scrambler Pro

Mid-Long Range Set:
TAR21 with Holographic Scope and FMJ
Bling Pro
Stopping Power Pro
Steady Aim Pro/Scrambler Pro

With both guns, I use the AA-12 Shotgun as secondary, and Bling Pro allows me to equip that with a Red Dot Sight and a Grip.

Those are the only two classes I ever use, and I've probably amassed over 1500 kills with each of those guns. More often, I will go with the UMP45, because I feel it is more versatile, as it can shoot both close range and long range efficiently, as long as you make sure to account for recoil.

As far as killstreaks go, it really comes down to what you're comfortable with, and what you know you can achieve. Starting out, I just stuck with the 3/4/5 pattern, because didn't often get that far. With the two days of play time I have accumulated playing MW2 so far, I've become a much better player, thus, netting larger killstreaks easier. I've been using the Harrier Airstrike/Pavelow/Chopper Gunner approach recently, which pretty much allows for a 20 kill streak once you get to 7. The Harriers will usually take you straight to 11, if not farther, and the Chopper Gunner will easily net you another 9 to 12.