#1 - software stuff. I sometimes can't believe the amount of spyware & adware that's put on computers. When you first boot up, it literally takes 5 minutes for the hard disk to calm down even before you press any key. Get rid of all that stuff. Uninstall stuff you don't need. Disable stuff that starts when your computer starts (e.g., you'd see an almost endless list of icons on your system tray [by the time/date display]).
#2 - Hardware stuff. Memory won't help if you're not already taxing what you have now. The thing that makes a computer slow due to low memory is because of access to the hard drive. This is called virtual memory...when you don't have enough "physical" memory, an OS will remove some things you don't need in memory at the moment, store it to the hard drive, and then load what you do need from the hard drive, hence "virtual".
A 32-bit CPU can address about 4GB of memory...i.e., it doesn't depend on how much memory you actually have installed. Now if you have 1MB of physical memory, and you run a program that takes up 10MB, the OS is gonna constantly transfer data between physical memory & your hard drive. It's like this: the OS maps that 4GB memory address space to a physical address (keep in mind you really only have 1MB to work with, so that other 3.99GB is mapped to a space on the hard drive). What happens when you try to access an address that doesn't physically exist in memory? You get a page-file error (or paging fault). Then the OS kicks in: it sees what address you were trying to get at, sees where it exists on the hard drive, sees what stuff it doesn't need in memory at the moment, "swaps" the data you don't need in memory with the data you do need from the hard drive. Then your program continues running. That's why if you were running a memory-intesive program & then alt-tabbed to a different program, it'll seem like it takes forever for the new program to come up. A hard drive is a VERY slow device when you're talking clock cycles. Speaking along those lines, grab a faster hard drive (7200+rpm). This will help a little but nothing signficant. So all I'm sayin' is this: don't grab 1TB of memory if you don't need it. If you're not doing graphic design or video/sound editing or whatever you don't need that much. 512MB is a lot.