[fallen1]

just wanted to add one more post...

Windows XP (and 2000, and technically 95/98/ME, but thier memory ustilization is so terrible than between 512MB and 1GB there is little performance improvement, and over 1GB is a waste of money) can recognize up to 4GB of memory in total. However, the total is made up of more than the sticks of RAM you have. Even though it doesn't show up in your system stats as such, Windows also counts the RAM on your video card and memory allocations for certain input/output functions and controllers. If you but 4GB of RAM in your computer and have a 256MB video card, Windows will only recognize about 3.5GB of RAM because it counts the 256 of your video card and around 256 more in overhead that is dedicated to certain controllers and processes. Windows XP 64bit edition with an Intel Itanium processor solves this problem by upping the supported memory limit to 16TB (as in TeraBytes). but good luck getting your hands on that (and that kind of hardware). Also, as of now the memory limitations of the upcoming Windows Vista will depend on the particular version, with the basic version having the same 4GB limit as XP, and the Business, Enterprise, and Ultra versions supporting up to 128GB of RAM.

Now back to the "how much is enough and how much is too much?" part of the discussion. 1GB will run XP and most applications just fine, so long as you don't over do the multitasking. 2GB is the "sweet spot" beause it allows you to fill all the RAM slots of your computer with matching sticks of RAM so that you get the best dual-channel performance possible, without going over the 3.xGB limit of what XP can actually utilize, which would result in a decrease in how efficiently your system can utilize the RAM. for example, if you have 2 RAM slots the most you can put in without going over the 3.xGB limit is 2GB (1GB in each slot). if you have 4 slots you would use either a pair of 1GB sticks or 4 512MB stick of RAM to keep everything running in optimal dual channel mode. You could put 3GB in by using a pair of 512 sticks and a pair of 1GB sticks, but most motherboards don't support dual channel mode unless all the sticks are matching (and those that support such seperate matching pairs tend to not do a great job of it).
If you install 1.5GB of RAM you are either using mismatched RAM (a 1Gig stick and a 512 stick or two 256 sticks and two 512 sticks) or an uneven number of sticks (three 512 sticks) in which case you loose the benefits of dual-channel mode and will actually have lower performance than with a matched pair of 512 sticks running in dual channel mode.
One thing about dual-channel mode to keep in mind is that the sticks have to match bothe the sixe and the timings (CAS latencies) to run properly in dual-channel, so it's a good idea to get the same model of RAM for both if you're not buying it as part of a dual-channel kit. You can usually use RAM that has matching sizes and CAS latencies from different maufacturers, but some people have run into problems with certain models of RAM from on manufacturer not wanting to play nice with certain models from other manufacturers.