A culmination of processor, graphics card and memory will determine end performance. Not just RAM which is a common misconception. I only have 1GB ram, and it runs silky smooth, but combined with a more recent processor (though not current), a fairly decent video card , high speed hard drive and decent motherboard work together to produce a decent experience.
As painful as it sounds, try this, reduce all the graphics settings to minimum and uncheck all the effects features. THen by process of elimination, move the sliders forward to higher settings and try the game again and check the performance. keep doing this until you hit a point where you are happy with how it looks. Don't even consider the checkboxes yet. If you can get them all to full, and you are happy with things, then one by one, check off the boxes, and test your performance in game.
Unfortunately, video card manufacturers prey on the fact that most users equate memory size of the video card (i.e. 32mb, 64mb, 128mb, 256mb etc) with performance of the video card. It was only by trial and error, and many uninformed purchases, that I learned that GPU (Graphics Processor Unit) speed, pixel pipelines (Important!) Shader version (very important for advanced graphical effects like post processing, reflections, enviromental effects, etc.) have a much greater impact on performance than how much texture memory ( the MB size your vid card has) can handle.
To summarize, its more important what your vid card can do with all that memory, than how much memory you have. This is why you will find some high end cards with only 128 MB of memory that cost up to 5 times as much as a "starter video" card with 512MB of memory. There are whole diatribes on the internet about these inconsistancies, I'll let you research them on your own. Hardware manufacturers are MUCH more sneaky than software developers!
Hopefully this will help you, and give you more insight into the evils related to graphics hardware!
But please remember, I only speak from personal experience and my own opinion. There are people here that are much more qualified to answer in more detail and more accuracy!
Cheers!