It makes no performance difference, but make sure high density modules (8 chips) are compatible with your motherboard before buying them. Unless you have a very old motherboard, it should be compatible with high density modules. High density modules are less expensive to assemble because they require half as many chips.
Its a MSI Ultra II-C motherboard running a VIA KT333 chipset.
I'm also trying to figure out how much difference CL 2.5 to CL 3.0 makes. The cheap sellers on eBay all seem to be selling CL 3.0 PC2700 DDR.
I've never had a stick of RAM bigger than a 512mb (and its blown, currently running a 256 stick) but they were always CL 2.0 and CL 2.5's. I've never tried 3.0 and not sure how big of a performance hit I'll take if I settle for the cheap one that is easiest for me to afford.
Crucial don't provide clear specifications, but they should be low density like the Kingston module. It's up to you, but I'd buy modules that are guaranteed to work with that motherboard (or at least are low density, i.e., 16 chips).