Here's a vague description of what you want for $650 USD. Try to make it fit for yourself in NZ.
A z68 or p67 motherboard. Look for UEFI instead of bios. Good manufacturers, in my order of preference would be Asus, msi, biostar, gigabyte (no uefi). Others exist, price is the big factor here.
An intel Sandy Bridge processor. minimum is i3 2100. maximum is i5 2500k or e3-1220 (a server processor). Buy this last, make it fit your budget. Try to avoid buying simple higher clock speeds, if a lower clock speed exists and is significantly cheaper, get it. $20 for a 5% increase in speed is not worth it.
Any DDR3 ram you can find at the cheapest possible price. Try to balance speed with the price, but it's not important. Heat spreaders do nothing except increase the cost and look cool.
Any HDD you can find at the cheapest possible price (look around for cheap ones from any store, doesn't matter if ebay or whatever). Don't try to get huge storage capabilities. Save up for an SSD later.
Any manufacturer graphics card of: 6870, gtx 560 Ti, 6950 (1 GB or 2 GB). Balance this with the CPU so that you fit your budget.
PSU - look for decent manufacturers (Antec, Corsair, Enermax, Super Flower, Seasonic, a few more) first.
Double check your choice on the PSU review database for its authenticity and likelihood to break/destroy your computer. depending on the graphics you choose, you will want anywhere from 450W to 650W. 500W should be able to run everything just fine, but personally I would stick with a 500w, 6870, i3 2100 or some other i5.
Case - get something you think looks good and fits your budget. If you go super super cheap, you won't be disappointed. If you spend $100 on a case and it's not 100% perfect, you will be disappointed.
This is actually the generic metric I go for when I create a build. The more money you have, the more you can spend on each part, making the overall computer better, but not linearly faster. The minimum computer you should be buying is an i3 2100 with a 6870. the maximum you should be buying is an i5 2500k with a 6950 2 GB. That goes for
all price ranges. If you want to go cheaper you should save up longer. If you want to go more expensive you need to get a life.
If you really must go cheaper, then the AMD route is the way to go with a 6850.
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