On the purchasing side, if it makes a difference the cheapest (and sometimes by far) place to get mem is
www.computergiants.com they have Kingston 2GB Kit 533MHz DDR2 PC2-4200 DIMM for about $270 and other 2 gb kits simalarly priced. That's about as much as 1gb at other places. If it makes it where you can buy 2gb now then no need to quarel over perfomence of matching over unmatched mem. Hope it helps