I'm not really sure that having more RAM will do much for latency at all. Latency is a function of the PC's ability to shovel audio in and out of itself, which is predominantly determined by the interface you are using.
Having more RAM saves the PC having to make more calls upon the hard drive (by loading stuff into memeory), and that generally makes, for example, recording and playback smoother.
Not directly, but if you machine is wasting a lot of CPU cycles sending data to the harddrives and back causing you to have to bring up your latency, you might be able to get away with a little lower latency by upgrading the ram.
I've been using 1 Gig RAM for a long time without experiencing latency problems. In recent times I upped it to 2 Gig. This had no effect on latency, though I did notice a more general performance improvement
More than anything else, your latency depends on your drivers. And when were talking about the lowest achievable round trip latency's absolute rock bottom limits, it is completely dependent on your drivers.
Correctly matched RAM that is compatible with your Motherboard and other RAM you already have installed never hurts (assuming your machine is able to use more than a gig). Mismatched incompatible or bad RAM can actually cause BSODs
Also what kind of latency are you asking about if it's sample playback latency being triggered by MIDI more RAM can help. If it's recording latency then more RAM won't make any difference at all
Actually it's latency thru plug-ins that I'm experiencing. And I get it during playback. I do use the feeze function on almost every track and that helps.