best game(s) 7,5$ can buy

SR3 maybe? but i recommend you to wait for more attractive sales.

Saints row 3 :D

If you wait until the last day of the sale, Portal 2 is a great buy if you don't already have it. Recently Fallout 3 and Fallout NV were on sale for 5 dollars each and there is a possibility they might be a daily deal soon (I especially recommend Fallout 3). Other than that, wait to see what catches your eye.

$7.50? I'd grab Star Wars: Knights of the Old Republic for $2.50 and save the rest for the next round of deals.

KOTOR + Broken Sword Trilogy + Patricians & Merchant bundle (can be found on the Darkstar One store page)?

Trine 2 !

GTA Complete Pack, but you need to wait few days(hours?)

Trine Franchise, Portal bundle, Dungeons of Dredmor, Legends of Grimrock, Mass Effect 1 & 2, Terraria, Might & Magic: Clash of Heroes

any of those are good choices, some depend whether they make the final day sale though

trine franchise or metro2033

Get Cave Story if you don't already own it, honestly there is always a bunch of games that are good and 2.50 so you could in fact get 3 decent games if you wait for the right ones.

The Orange Box (while it's on sale).

mass effect & mass effect2

orange box