First that would be an XP2800+ not XP2700+ as far as the cpu model number. For 2.2ghz unless Sempron that would be the XP3200+ seeing that stock cpu speed. For memory a matched pair of 512s would be ideal for the PC2700 memory there over mixing a 512 with two 256mb dimms.
As long as both brands are seeing the same type and not mixing single sided with dual sided memory or a timing difference you wouldn't see too much difference simply by brand or size while the ideal would be the matched pair of the same size and type. A look at the board spscifications to see what memory is supported however is generally the first thing done.
As far as the 100, 133, and 166mhz that refers to the front side bus speeds there. PC2700 DDR333 would normally run default at 166mhz in the bios. 2x166.5 = 333mhz in that sense.