According to Green Guide 1.6 tons of raw material are required to make a single computer. On top of this pre-purchase impact, there is the impact of the computer once discarded to think of as well.
Consider buying a second hand computer. Due to the nature of computers you may want a warranty so find a store such as Laptop Universe who deliver nationwide. We bought a laptop from them and it is great.
If you do want a brand spanking new computer check out this guide which rates computers from gold to bronze based on “…51 criteria including toxics reduction, recycled content, energy efficiency, ease of recycling, product longevity, the manufacturer's takeback and recycling program, and packaging. All computers are required to meet 23 mandatory criteria, and the ones rated 'Gold' unsurprisingly are the best.
When disposing of any computers, or computer related parts, consider donating it to an organization like Molten Media. They rebuild, refurbish, and recycle computers for resale, and in the process provide training opportunities and employment skills for young people.
For Auckland folk, there is a new company in Avondale, IT Recycle, who will pick up your e-waste as long as you have two or more items. What do they do with them, well the name is a giveaway :)