Have a look at these components:
Mobo:Asus M4A785TD-M EVO click here
CPU:AMD Phenom 550 click here
PSU:Storm 500 click here
RAM: Kingston 1600 click here
HDD:Deskstar 320 click here
GPU: MSI GTX 460 768MB click here which is faster than the 5770
Obviously you will need to factor in optical an optical drive and operating system , but should come in within budget.