I will be building a new pc soon but i am wondering whether I should get a Gigabyte GTX 650 1gb GDDR5 or an Asus 7750 1gb GDDR5
As I've said before, it depends on what games you play and how they are priced in NZ. These two will trade blows.
Here are AnandTech's benches:
http://www.anandtech.com/bench/Product/681?vs=535
And here is an excerpt from TomsHardware:
The Radeon HD 7750 is the fastest graphics card you can get right now that doesn't require an auxiliary power input; it draws all that it needs from a 16-lane PCIe slot. If you're upgrading an older machine with limited power supply capacity, that's an attractive point to consider.
And it's a decent performer, too, particularly in light of AMD's most recent Catalyst driver packages. The GeForce GTX 650 delivers comparable performance, but its typical $120 sticker price is too close to the higher-performing Radeon HD 7770.