if you do decide to build, here what i was able to put together to fit your budget as best i can.
http://www.newegg.com/Product/Product.aspx?Item=N82E16812123159 sata II cables x2 $4.58 (cables for HDD and Dvd burner)
http://www.newegg.com/Product/Product.aspx?Item=N82E16827106289 LITE-ON Black 24X dvd burner $19.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136075 Western Digital Caviar Blue WD1600AAJS 160GB $39.49 (if you need more space, you can up that for just a few $ more.)
http://www.newegg.com/Product/Product.aspx?Item=N82E16811129084 Antec Two Hundred v2 case $49.99 ($37.99 after $12.00 Mail-In Rebate)
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371035 Antec EarthWatts 500W $74.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150448 XFX HD 5750 $129.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231253 G.SKILL DDR3 1333Mhz 4GB (2 x 2GB) $103.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16813128431 GIGABYTE GA-770T-USB3 AM3 motherboard $79.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103847 Phenom II x2 3.1Ghz 550
http://www.newegg.com/Product/Product.aspx?Item=N82E16832116754 Microsoft Windows 7 Home Premium 64-bit (OEM) $99
Subtotal: $690.99
Shipping: around $25 (for me it will be if i got this, shipping would be $25.21)
Total: around $ 715.99
With Rebates: around $ 703.99
Now, If you need a keboard, monitor, mouse, speakers, ect, those will cost extra. Which will mean the iBUYPOWER pc will be the better pc to get.
If you just need the desktop, the pc i manage to put together (which was a little difficult to fit with the budget) will do very nicely and if in the future if you need more performance like addtional cores you :
A. can swap cpus with to a quad core or a hex core. Examples, like the Phenom II x4 955 or Phenom II x6 1055T
B. Have a chance unlock the 2 disabled cores. Thats if the motherboard supports this and the other 2 cores are not fully defective.
Now if you watch for a little bit (say a day). You'll probably see people will find better parts for a better price than i listed.
Although This can give you an idea what you can do with around $700 building it your self.