If you work with large spreadsheets or large files Adobe Photoshop, Premiere Pro, After Effects, or modeling software like AutoCAD or Maya, then you will notice a significant performance boost. Especially if you run a lot of applications at the same time. But if you are just gaming, browsing the Internet, or doing homework then you won't notice anything at all.
Remember, more RAM (I'm assuming all modules run at the same speed/timings) only speeds up processing by way of avoiding page faults. Unless you actually USE all of that RAM at a certain point, more of it will have no impact whatsoever.
Unless you do something like Adobe Photoshop, encoding, or whatnot, 4GB will still suffice. Really, 4GB is still consensus for non-Tri Channel based systems (where 6GB [2GB + 2GB + 2GB] is the most logical setup)