the ASUS A7V333 and ABIT KX7-333 offer the same performance. both are great brands. the ASUS offers onboard 6-channel sound (not bad quality at all, but i prefer audigy), usb 2.0, and possible raid. don't really consider the KT7A-RAID as it is a kt266a motherboard of yesterday.
333mhz (pc2700) ddr is 25% faster than 266mhz (pc2100)... do the math. don't consider getting a pc2100 memory wth a kt333 board as it would almost defeat the purpose. and 1x512mb dimm is no faster than 2x256mb dimms. they're the same.