actually i remember reading a review for a p4 chipset (by via) that used single channel memory, which was compared to intel boards that used dual channel (this was back when the 800fsb p4's were out and about). if i recall correctly, the via chipset kept pace very well with the intel boards, despite its "limitation", of course they were using ddr400 ram. i mean, im pretty sure the intel boards did better in memory benchmarks and such, but in real world apps the via board kept pace very well. so, i doubt it matters with a 533fsb p4.
if it were me, i'd go 1gb in 1 slot or 2x512 in 2 slots, whichever's cheaper. since the dual channel won't really matter it wouldn't bother me that much, but it might you so if you care that much go with the 512's. also, assuming you have 4 dimms (since you asked 4x256), this way you can fairly easily upgrade to 2GB. might be cheaper to go the 512 route since a lot of people have 512mb ddr sticks that they don't use anymore (am2 and lga775 upgrades), might be able to pick some up for cheap from them.