If you already have DDR2 memory and a decent motherboard, upgrading to DDR3 isn't necessarily a good move as your system's performance will improve marginally at best. There are few socket 775 DDR3 motherboards.
Then read the reviews and get the one that seems to be the most stable and/or has the best reviews. It's much easier to recommend excellent socket 775 motherboards that use DDR2 memory like the Gigabyte GA-EP45-UD3P or the Asus P5Q Deluxe.
Don't get the DP45SG. They have been known to have a couple of issues -- one with sound, and another with being unable to use anything other than 1.5V RAM. By which I mean, in a lot of cases, if you have memory that takes a different voltage ... 1.6V, 1.7V, you name it ... you would normally just go into the BIOS and set the voltage manually, but this board WILL NOT let you do it.
Or more accurately, it'll let you adjust the voltage, but then when you boot up, it'll screw up with power distribution and the whole system won't turn on. For example, look at this thread on the Intel site full of people encountering this same problem, and an Intel board engineer even shows up in the discussion, and the conclusion that they reach is that the board is simply not designed to accommodate the majority of DDR3 out there. Intel boards are kind of known for not being friendly to "performance" components, or basically anything that's not standard-issue stock settings. There are a whole series of threads like this:
DFI ... they've got an OK reputation as far as I know. Not great, but not bottom-of-the-barrel either.
Personally, for a socket 775 board with DDR3 support, the one I liked best was the ASUS P5E3 Pro. Not a really well-known board, but I've got one in my quad core machine and couldn't be happier with the stability and overall performance...