Assuming your copy of Windows Vista is a retail version and did not come in a system you already purchased (like a Dell or HP), then you should be able to install it on a new computer which means $600 is actually enough to build a decent system. If Windows Vista is on installed on a Dell, HP, Acer, Lenovo, or whatever brand name PC, then it cannot be used because it will only work with the specific motherboard that the PC has.
I recommend the following part from Newegg which works out to $582. That excludes shipping charges (if any for specific parts) and also taxes (if you live in a state where Newegg is headquatered or where they have warehouses).
An important note a few of them also have promo codes which reduces the price even more. However, most of the promo codes expires today; I supposed by 11:59pm PST. Therefore, you need to decide quickly if you want to take advantage of the promo codes. Some also have rebate cards as well.
AMD FX-4130 = $104 + Free Shipping
+ $10 off w/ promo code EMCXLXT26, ends 9/9
http://www.newegg.com/Product/Product.aspx?Item=N82E16819113291
ASUS M5A97 LE R2.0 AM3+ AMD 970 motherboard = $85 + $6 Shipping
$10 Rebate Card.
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131872
G.SKILL Ripjaws X Series 8GB (2 x 4GB) RAM = $72 + Free Shipping
+ 10% off w/ promo code GSKILLBTSSEP, ends 9/14
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231538
SAMSUNG DVD Burner = $20 + Free Shipping
+ $5 off w/ promo code EMCXLXT226, ends 9/9
http://www.newegg.com/Product/Product.aspx?Item=N82E16827151266
Rosewill Blackbone Black Steel Case = $45 + Free Shipping
+ $13 off w/ promo code EMCXLXT43, ends 9/9
http://www.newegg.com/Product/Product.aspx?Item=N82E16811147023
CORSAIR CX500M 500W PSU = $70 + Free Shipping
$20 Rebate Card
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139050
SAPPHIRE 100354OC-2L Radeon HD 7870 GHz Edition = $185 + $6 Shipping
$15 Rebate Card
+ 2 Free Game
http://www.newegg.com/Product/Product.aspx?Item=N82E16814202025
System Cost Excluding Shipping / Rebate / Promo Codes = $582
Shipping Costs = $14
Total Cost excluding Rebates / Promo Codes = $596
Total Rebate Cards = -$45
Total Discount from Promo Codes = -$35
Net Total Cost = $516