Buying a Laptop/desktop for college?

Ok so Im a college student going into computer engineering. I definitely need a laptop for typing notes and essays but I also kinda want a desktop for security. Problem is I only have about $520 ! Im ok with buying refurbished or used laptops or desktop online so here are my options:
1.Buy a Laptop thats used but still ok
2.Buy a cheap netbook and a cheap desktop
3 Buy a good netbook

Which option should I go for?
Also are netbooks good enough for college let alone engineering?
What worked best for you?
Can you recommend any computers ?

Here are my dream specs( I understand that I’m asking a lot):

Laptop

+250gb hardrive
+2.0ghrz processor
+2gb ram

Desktop

+100gb hard drive
+2.0ghrz processor
+2gb ram

Netbook
Good disk size
fast processor
overall fast computer
Also long battery life are really important to me.

http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=4240174&CatId=2629
2.3GHz processor 250GB hard drive 3gb ram $300

http://www.overstock.com/Electronics/Dell-2.6GHz-Pentium-4-Desktop-Computer-Refurbished/2586732/product.html?cid=123620&fp=F&ci_src=14110944&ci_sku=10800030

$169 2.6ghz prosessor 512mb ram 40gb hard drive (refurbished)

http://www.amazon.com/ASUS-1005HA-PU1X-BK-10-1-Inch-Black-Netbook/dp/B002DYIXMI/ref=sr_1_1?ie=UTF8&s=electronics&qid=1253657413&sr=8-1

Netbook with 1.66GHz processor, 1gb ram, 160gb hard drive 10.5 hour battery life $354 free shipping

Buy one save money for the next one. OR buy this

http://www.newegg.com/Product/Product.aspx?Item=N82E16883103222

Dual core processor (2.66 x2) 4gb ram, 750gb hard drive $499 free shipping

http://www.cyberpowerpc.com/system/AMD_Athlon_X2_Configurator/ also a good deal, but they take a while to ship

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