Learning to build a system MKII

APPROXIMATE PURCHASE DATE: in a month or two
BUDGET RANGE: AUD1,000 to 1,200

SYSTEM USAGE FROM MOST TO LEAST IMPORTANT: word processing, pdf, statistical calculation, photo editing, watching movies, web browsing

PARTS NOT REQUIRED: monitor, speakers,

PREFERRED WEBSITE(S) FOR PARTS: no preferred website
COUNTRY OF ORIGIN: Australia

PARTS PREFERENCES: I would like to use Intel cpu

OVERCLOCKING: Maybe later
SLI OR CROSSFIRE: don’t understand what it is, anyone can help explain

MONITOR RESOLUTION: not sure, but it is a 19” LCD about 1024x768 or 1280x1024

ADDITIONAL COMMENTS: need to run many word processing, statistical packages, pdf programs all at once.

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so far I’ve selected the following parts

CPU:
Intel i5-750 Core i5 CPU, 2.66 GHz, 8MB L3 Cache, Socket LGA1156
http://www.newegg.com/Product/Prod [...] -_-Product

Motherboard:
GA-P55-UD3P
http://www.gigabyte.com.tw/Product [...] uctID=3163
or
Asus P7P55D-LE Intel P55 S1156pin DDR3 Motherboard with Express Gate
http://msy.arpatubes.net/display.p [...] uery=26580
http://www.asus.com.au/product.asp [...] templete=2
or
Asus P7P55D-LE P55 Motherboard Raid DDR3 GLAN LGA1156
https://www.pcdiy.com.au/advanced_s [...] cadf16f363
or
Gigabyte GA-P55-UD4P
https://www.pcdiy.com.au/product_in [...] cadf16f363

Video card:
Gigabyte ATI HD4670 PCI-E 2.0 1GB
http://www.gigabyte.com.tw/Product [...] uctID=3059

Hard disk:
Seagate 1TB SATA2 Hard Drive 3.5" 3Gb/s NCQ HDD - 7200rpm, 32MB Cache
http://www.seagate.com/ww/v/index. [...] ee0a0aRCRD

Ram:
Corsair TW3X4G1333C9 4GB (2x XMS3 2GB) PC-10600 (1333MHz) XMP DDR3 RAM, 2x2...
http://www.digitan.com.au/product. [...] t=0&page=1

DVD burner: SATA Burner Pioneer

PSU:
COOLER MASTER Gladiator 600 RC-600-KKN1-GP Black SECC Body ; Mesh Front bezel ATX Mid Tower Computer Case
http://www.newegg.com/Product/Prod [...] 600%20Case

hi everyone
I’m a newcomer to this, building a system. I’m learning as doing it. So far I’ve chosen the above components. My main questions are:
• Are the two ASUS motherboards the same thing?
• Which motherboard is the best buy? If none of them, any recommendation?
• Is the video card good enough for my purpose? I know some may suggest it is not good enough for the system overall, if not any good recommendation without breaking the bank? I’ve considered GTS250 1GB but it is a bit dearer. Anything in between?
• The PSU is really a headache for me. Is the one above good enough? Or any other recommendation? Reasonably priced and just enough for the job? If case and power supply are bought separately, please recommend both case and power supply? But I notice it is more expensive to buy them separately!
• I don’t know if I need any cpu heatsink or cooling, as I learn if cpu comes with a box then there should be a heatsink good enough for the job, so I don’t need to buy any extra, is that true?
• Also I need a wireless network card, any suggestion? does the motherboard come with one?
• Can anyone explain what SLI / Crossfire is?

Sorry for asking all the dump questions, but your help is much appreciated. Thanks so much for your advice in advance. Remember, I’m a newcomer, if any jargon is used, please elaborate a bit.