Take a look at this site if you want to look for a prebuilt:
then select the following options (most are the defaults):
COMPUCASE HEC 6C28B Black Mid-Tower Case, No PSU, ATX
HEC Orion HP 585D 585W Power Supply, 24-pin ATX, ATX12V, Dual 80mm Fans, OEM
GIGABYTE GA-MA785GMT-UD2H (rev. 1.0), AM3, AMD® 785G
AMD Phenom™ II X3 720 Triple-Core 2.8GHz, AM3
CRUCIAL 4GB (2 x 2GB) PC3-10600 DDR3 1333MHz CL9 1.5V SDRAM DIMM, Non-ECC
SEAGATE 500GB Barracuda® 7200.12, SATA 3 Gb/s, 7200 RPM, 16MB cache
SONY AD-7241S Black 24x DVD±R/RW Dual-Layer Burner w/ Lightscribe, SATA, OEM
MICROSOFT Windows 7 Home Premium 64-bit Edition, OEM
WARRANTY Silver Warranty Package (3 Year Limited Parts, 3 Year Labor Warranty)
This comes to a total of $603 and you can drop in any GPU you want, like a 5770. You can also play around with the configuration options to play around.
For the best bang for your buck, I would recommend using only one GPU. Skipping SLI or Crossfire allows you to use a less expensive PSU and MoBo as well as only buying one good GPU instead of two mediocre ones.
The AMD X3 can also be unlocked to a quad core giving you great bang for your buck.