You realistically don't need more than 4 gigs of ram, 6 gigs is the "sweet spot" for most programs, which means that even though you have 8, it will be, for the most part, wasted money on the excess. As programs become better optimized for 64 bit computing, you might find a use for more than that but at the moment this is the fact.
However, as the above user stated, the price is so minimal that you can easily go with an 8Gb kit and not feel the burn on your wallet, even though you won't be truly getting the performance of the whole kit, you'll still get your money's worth on a mid range kit.
At this juncture, 16 Gb is far in excess of what almost any program out today can use.
4GB is fine for most users unless you are in some heavy video rendering/converting or something, but since Ram is rather inexpensive at the moment, if you can budget it get 8GB. more ram should = less swap file usage.
I would say 6GB with room to expand, especially if you are running indicators. A lot of times the amount of tick data you have will require more RAM. I am also a trader and I have 6GB and happy with it.