Wait you use DDR2 RAM?
Should have told us that in the first post :P
Also the 5770 should run close to 100% on a PCI-E 2.0 X4 slot as it uses much less bandwidth than a 5870.
Here's a motherboard that use DDR2 memory and fits the specs you were asking about.
Newegg.com - ASRock A785GXH/128M AM3/AM2+/AM2 AMD 785G HDMI ATX AMD Motherboard - AMD Motherboards
If you want to get an AM3 motherboard and switch to DDR3 RAM though, you could just sell your DDR2 RAM off for enough money to buy some DDR3 RAM.