Times are changing, the land of hardware renderers using the GPU to render your images instead of the CPU is getting closer every day, check out Iray by Nvidia as one example.....This is the future of rendering imho and will eventually be the standard way all renderers work, it makes so much sense to load the renders onto a GPU....still a while away before they really nail it but worth a thought when considering a new graphics card.

Maya 2015 can use both openGL and also DirectX11 so the days of pushing quadro cards and firepro is only going to be for the very top end systems were talking "the best"....these cards are not cheap for a decent one!

You will be able to interactively move around a viewport with any high end gaming card in Maya, I have done this for a few years with heavy scenes + I saved some cash in the process....but to be fair with a very heavy scene even with the best graphics cards you would use use common sense to optimize a scene that makes the viewport more workable with things like bounding boxes , view selection , hiding objects until render, render layers etc instead of trying to manipulate a really high poly count scene all at the same time.

You mentioned memory, I do a great deal of dynamic destruction effects, explosions and fluid/liquid effects and if that's something you will be getting into (great fun) you will want to look at as much memory as you can get your hands on 8G is very usable for many scenes until you start getting more intense along this path then you might want to look at considerably more 16G, 32G, even 64G+ sometimes as it can be really memory hungry with a few simulations running, especially now with Bifrost + Bullet + NHair and fluid effects + Ncloth and NParticles