If you were running Windows XP X64 I would say you did not need to add more than 4 gigabytes unless you were playing Multi-Player Games or boxed computer games, using CAD software or wanting to create a Server system for a larger network.
Windows Vista on the other hand requires much more resources than does Windows XP. As one person commented it is truly difficult to say how much is too much for you when we don't know your habits. If you simply perform normal everyday tasks such as browsing the web, using Office software, and play,edit,create, and burn multimedia files I would say that even for a Vista machine 4 gigabytes would be plenty because unlike in a X32 based OS the X64 recognizes and is able to utilize all 4 of those gigabytes. An X32 system is lucky if it recognizes and utilizes 3 out of 4 gigabytes, but video memory and other onboard memory can make up a bit of the difference, while in a X64 OS it ADDS to the Physical RAM installed up to a limit of 8 gigabytes in most motherboards intent for home or even office uses.
A motherboard created with a Server purpose in mind on the other hand could recognize and utilize up to 128 gigabytes of Physical RAM. Of course, additional CPU, cooling, and Power Supply wattage would also be required to support that type of setup.
I don't agree with the individual who said additional RAM is always preferable. That simply isn't accurate. If you do not utilize hardware resources they are wasted, and therefore your money is also wasted. As bigger, and more powerful CPU's are created and released to market I also see this issue. Okay, so Intel now creates a Quad Core Central Processing Unit or CPU. Who in a regular home usage would ever utilize that much power? Even setting up a Windows Home Server a Quad Core is simply overkill and a waste of money. It would be better and far more efficient as well as save money to buy what you will actually utilize and then a tiny bit more for insurance of not maxing out resources.
While it may be cool to have a big bad super powerful knock your socks off machine, if you never actually get it into high gear, but rather drive it only in second gear, when it has five gears, then you bought far more car than you will ever use and wasted a ton of money. Now, for the car scenario having too much car can have other applications, such as impressing people. Computers on the other hand seldom impress to a point it is worth the extra money on components and power never to be utilized. I am done with the car analogy. LOL
You are smart to ask this question rather than run out and buy additional hardware before finding out if you actually require it. With a X64 machine and only using it in the normal example given above buying and installing another 2 gigabytes of RAM would be a waste of your money. In this economic crisis there may be plenty of sales going on but a sale is only good if you actually use the items you purchase. Smart purchases and expenditures are a sign of future financial security. You sound like a very intelligent individual.
Only you know how you use your system and any future plans you may have for it. If you have plans to create your own website and wish to host it yourself on a Web Server then I would recommend buying extra RAM now while it is on sale and then install it when you are putting together your Web Server. The same goes for other instances of higher technologies which require more power. It is a great time to estimate what future needs are and buy today at reduced prices and sales prices. Even with technological advances today's RAM modules will not be obsolete any time soon.
I read the post about Registry Boosters/Cleaners/Optimizers and I strongly recommend you not run any of these types of utilities on any system because the long range damages far outweigh any short term gain in performance. It is far better to get into the habit of only installing software you know you will use so you do not need to uninstall any than to install, uninstall a lot of software. This is the #1 cause of errors in the registry and a bloated registry filled with obsolete keys and values. By all means clean your trash files off of your disk with tools like the free CCleaner, but don't use its Registry Cleaner. Please take my hard learned lesson on how much damage these types of tools cause Windows. I had a fatal crash and had to reinstall my OS and because the system was less than a year old I was stupid and didn't back up irreplaceable files which I lost forever.