for $1500 u can get a ton of stuff
first off lets start with the case....a mid-tower may be able to fit everything but if u have the space i would suggest a full-tower case such as these: (i will use the cosmos for pricing)
http://www.newegg.com/Product/Product.aspx...9-043-_-Product antec 1200
http://www.newegg.com/Product/Product.aspx...9-138-_-Product cooler master cosmos (189.99)
next u will need a motherboard: the one u chose will work, however it didnt have usb 3.0 or things to sort of future-proof ure pc like the msi board listed below:
http://www.newegg.com/Product/Product.aspx...0-274-_-Product msi atx motherboard (199.99)
next the procesor...the one u chose should be more than enough for anything ure girlfriend wants to do (265.99)
http://www.newegg.com/Product/Product.aspx...-_-NA-_-NA-_-NA and now for the ram: looking for compatibility with ram: u will see the ram listed as 1333, 1600, etc. these just need to match ure motherboards specifications (which is listed) these are a type of timing raiting, so the bigger the number the better for example: a ddr3 1333 stick is slightly slower than a ddr3 1600 stick.....u will also see cas latencies listed and timings that look like 9-9-24 the faster the ram the lower the timings. u also need to make sure of what ure mobo needs such as dual or triple channel ram (which is also listed) so pretty much u just have to match the ram details to the mobo details and make sure everything works together
http://www.newegg.com/Product/Product.aspx...1-311-_-Product (149.99) 8gb of ram (2x4gb sticks) if u buy 4gb sticks now it is easier and cheaper to upgrade in the future
the psu and motherboard u should never skimp on...on newegg if u search power supplies there listed on the left should be a wattage calculator...just fill it out and it gives u an estimate of what whattage u need, so get a psu a little over what the calculator says to be safe
http://www.newegg.com/Product/Product.aspx...9-009-_-Product (129.99) (850w psu should be more than enough for now and future upgrades u may make)
now u need a Hard Drive...dont know what u are using this pc for so i dont know how much to list so ill just list a 1tb drive which last u a long while:
http://www.newegg.com/Product/Product.aspx...6-284-_-Product (87.99)
and u also need an operating system: windows 7 home premium....just make sure u get the 64bit version otherwise u cant go over 4gb of ram! (99.99)
http://www.newegg.com/Product/Product.aspx...N82E16832116754and now the video card: hd radeon 5850 (309.99)
http://www.newegg.com/Product/Product.aspx...N82E16814102881u will also need a dvd/cd drive: i use this one and its really fast and lightscribe is great if u choose to use it
http://www.newegg.com/Product/Product.aspx...p;Tpk=hp%201270accept for monitor, keyboard, mouse set up this is all u need
the price for the things listed above (remember i chose the more expensive case to price) is: w/o shipping: $1463.91 w/ shipping(just chose it to my house to get a estimate) is: $1492.61 shipping= $28.70
ps. if u choose to get a cheaper case or cheaper other components i would suggest using the saved money to buy an after-market cpu cooler such as one like this...especially if u plan on overclocking anything
http://www.newegg.com/Product/Product.aspx...3-055-_-Product (59.99 w/ free shipping)
hope this answers some questions and helps u choose some parts...remember have fun with it!!!

good luck!