Firstly - the voltage indicates how much charge a particular memory module can take comfortably. Most components in a PC can take +/- .1V tolerances, however, with three slots on your motherboard, I would keep within the tolerance set by the manufacturer 2.5V or less. It may not seem a lot .1V extra - however, if you repeat that across three RAM slots that .3V can make a difference dependening what other components are on your board. Exceeding tolerances can burn out components or be a potential hazard - I stick within the guidelines, this saves me money and doesn't invalidate warranties.
Secondly, my PC uses DDR DIMM SRAM 184 Pin PC2700/3200 @ 400 MHZ, very similar to yours. Determine how much memory you want and how much you want to spend - dabs sells 512MB RAM to your PC's specifications quite cheaply, about £37.00 plus delivery if I remember.
If you want to use new RAM with existing RAM, make sure that the speeds i.e. PC2700/3200 match. It's not imperative but if your buy PC2300 and have a 2700 stick, then the new memory will run at the 2700 speed.
Lastly, I always use the specifications set by the manufacturer of my PC's. Although the crucial.com states one list - such sites do not give definate answers, they are calculations based on what information you enter. The manufacturers do extensive testing to give you the specifications. Personally, and others may disagree, I'd keep with these.
This way your PC warranties stay valid and you still get the benefit of an upgrade. A friend of mine only recently blew his board because he didn't fit extra components to specifications.
Hope this helps!