I have the Dell Inspiron 1501, Turion 64 x2(TL-56), Windows 7 Professional (64-bit), and 4Gb of PC-5300 ram installed. I had Vista Ultimate (64-bit), and while it worked reasonable well, it regularly consumed 2-3 Gb of RAM for no real reason. When I bought the laptop it came with 1Gb or RAM. I upgraded it to 2Gb as prices came down. I traded the 2Gb and some cash to a friend for a 4Gb set he bought that didn't work in his laptop. As the memory installed went up, so did the amount Vista used for itself. When i had 1Gb installed, Vista would take 800 Megs for itself. When I had 2Gb, Vista would use 1.3-1.5Gb. When I went to 4Gb, like i said above, Vista took 2-3Gb. As I type this, Windows 7 is using 40% of 4Gb, or 1.6Gb.
Bottom line - If you have an Inspiron 1501, I would STRONGLY recommend putting 4GB of RAM in it. This should be fine for most users and at this point is not an expensive upgrade that will dramatically improve performance. As soon as prices drop to around $200, I'll be putting 8GB in my laptop because I've read it works and I do CAD work on it.