RAM - Low Density & High Density? Confused...

[Redhatcc]

We are looking to order some RAM from an online supplier for cheaper than what we have it now here at our store... but someone saw this site and where it kept track of differant prices across the www but... when we went to purchase the cheapest ram (generic) it poped up that this density ram will not work for such and such and bla bla bla...

http://www.pricewatch.com/system_memory/ddr_pc2100_512mb.htm

(^^^ the site)

And i was wondering... we have alot of used sticks of ram we use here at the store and i've never realy had a problem with ram not being compatible... but is there a differance in compatibility with computers working with Low Density ram and High Density ram? Which would be the best or is there realy a differance? What would you suggest we order, the cheaper RAM (even though it has a lifetime warranty), or the most costly ram? Keep in mind we would use it here at a computer store on new and old computers all day so compatibility would be somewhat a key (and profit lol).

Thanks Fellow MSFN'ers