Sometimes you can mix the two. Sometimes you can't. Save yourself the headache and just get ECC memory. You can get used ECC memory dirt cheap on eBay. You can find 4GB and 8GB DDR2 kits for $10 including shipping. You're lucky your board supports ECC memory. It is so much cheaper on average. Because it gets pulled from used Enterprise equipment. The market is flooded and there is little demand. I recently threw out a pile of 1GB modules because it wasn't worth the effort to sell.
SK Hynix, Micron (crucial), Samsung and Toshiba are all good brands. Those are what you'll usually find. As they are the only RAM manufacturers. The likes of Dell, HP, &c buy directly from them. All those other brands you see just rebrand the memory and...