Go for DDR2 RAM definitely,because teh difference between DDR2 and DDR3 isn't much (and DDR3 isn't worth the money)
Also why it must have a PCI-E 2.0? Do u want SLI/CF? if so then go for a 780i board (for SLI) or X38 and X48 (For CrossFire) otherwise go for a P35 board.
For RAM go for a DDR2 800 RAM with 4-4-4-x or lower timings.
What kind of video card do you have? Do you have one, or two?
What kind of PSU do you have?
GA-EP43-DS3L - $100, one PCI-E 2 slot
GA-EX38-DS4 - $225, two PCI-E 2 slots (Crossfire), RAID
If your video card is an nVidia, get the DS3L, don't try SLI.
If it's an AMD, get the DS4 and do Crossfire now or later.
The difference is that Crossfire on X38 is much safer than SLI on an nVidia chipset.