Hi!
For 600 Dolars you can build pretty much a whole new system.Except the old case DVD burner and HDD.
Here's what i recommend you buy.Its all from newegg and its just an incredably good deal.
CPU - AMD Phenom II X4 955 C3
This CPU isnt a very new model but its still more than good enough for everything you throw at it.It's a very good overclocker too!
http://www.newegg.com/Product/Product.aspx?Item=N82E16819113007
Graphics Card - MSI HD 7870 Twin Frozr/OC
This graphics card will allow you to play almost every game at its maximum.It also features a great cooler, so it runs very cool & quiet.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127662
Motherboard - MSI 890GXM-GD65
Again, not a new model but it supports AMD Bulldozer CPU's so you shouldnt worry about anything if you want to upgrade your CPU later.
http://www.newegg.com/Product/Product.aspx?Item=N82E16813130269
Power Supply - OCZ ModXStream PRO 600w
At that price its by far the best PSU you can get.It can provide more than the power you need if youre going to overclock.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817341017
Memory - G.SKILL ARES 2 x 4 GB 1600MHz CL9 1.5V
High performance memory at an affordable price.Its more than what you need really but its a very good deal.
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231544
SSD - OCZ Vertex 3 60GB
This SSD isnt a new model.But it was a top model!Its one of the fastest SSD's avaible and its priced very well.If you dont know what an SSD is, its like a very fast hard drive with low capacity.Make sure you install your operating system on it (your Windows).It will make everything you do on your PC faster.It will boot up faster load faster and it will also become more responsive.
http://www.newegg.com/Product/Product.aspx?Item=N82E16820227737
CPU Cooler - ZALMAN CNPS10X Performa
It's the best in its class for AMD cpu's.It works very silently and will keep your CPU very cool.
http://www.newegg.com/Product/Product.aspx?Item=N82E16835118059
The total price for all of this is 596 $ after rebates.This is an extremely fast PC that will max out almost every game at a very low price.
hopefully ive been helpfull and have fun with the new PC/PC Upgrades