I've found that system RAM is pretty much the one thing one needs to worry about. I run EQ2 on an Athlon 2400, 1 Gb Ram, Geforce 5900FX, and I have no frame rate problems... I do turn down some settings, like pixel shader, but I still get what I like to call "nice" graphics at 1280*1024.
(Note that my fan is broken, so I actually run a reduced clock on that card btw.)
My wife on the other hand plays on my hand-me-down, which is an Athlon also, with a Geforce 4200 also with 1 Gb RAM.. her machine acts almost as well as mine, at 1024, sometimes even at 1280 resolution.
While I won't get into the ATI/NVidia debate, I always like to check out
www.sharkyextreme.com for their guides on extreme gaming rig, and value gaming rig, and choose from there. I have always been satisfied with the results, using the value gaming components.