Please help with system build!

Is the $1000 budget including the components you've already got? If so then the total is about $903. Which leaves plenty to get an AM3 board, DDR3 RAM and a 5770, which while not the most desirable for that res is certainly better than the 5750. Perhaps another forum member will know how to get a 5850 into this budget.

Here is my suggestion for an AM3 board and DDR3 RAM.
Asus M4A79TD Evo $133, $10 rebate and in a combo with the improved Athlon Quad; Athlon II X4 635 http://www.newegg.ca/Product/ComboDealDetails.aspx?Item... $230 before rebate.
And this RAM: CORSAIR XMS3 4GB (2 x 2GB) DDR3 1600 (PC3 12800) CMX4GX3M2A1600C8 CAS8 1.65V £107 Which would only add $16.

Change the harddrives to either of these as they perform better/faster.
Samsung Spinpoint F3 500GB $60
Seagate Barracuda 7200.12 500GB $60

One of the ways that you might be able to squeeze a better card in is if you change the CPU to a tri-core (Athlon II X3 435/440), if you're going to play GTA IV and it is a high priority you will need a quad core,

Here is the 5770 I recommend. HIS H577FM1GD Radeon HD 5770 1GB $167 with $10 rebate.

If you change the card to the 5770 and get the Asus M4A79TD Evo and CPU Combo I make the build to be $957, or $663 if you don't include the components you already bought.
With my suggested board the second slot is actually fast enough to make crossfiring worthwhile, whereas on the board you originally thought of only has one PCI-E slot above x4 speed. x8 is the minimum for worthwhile graphics performance in modern cards/systems.