If you are running Windows Vista on old hardware, do consider investing in a USB flash drive to take advantage of the ReadyBoost feature.
I had been running Vista for close to a month on an old Pentium 4 2.4GHz with HT and 1GB of DDR400 RAM. Booting up alone took as much as 65% of my RAM (as shown on the resource meter sidebar gadget). Opening a couple of apps (e.g. Outlook, Word and Internet Explorer) and switching between them always take at least a couple of seconds and you can see the harddisk activity light going crazy during that time.
Upgrading the RAM is not an option in my case as the motherboard has only 2 DIMM slots and they are already filled with 2x 512MB in a dual channel configuration. Replacing them with 2x1GB modules will set me back at least S$140 based on today's market prices for PC3200 RAM modules. To me, that's not a cost effective solution since my hardware is more than 4 years old and any parts could fail anytime now.
So today I took the plunge, went out and got myself a 1GB Trek USB thumb drive from Giant for less than S$15. Came home, plugged it in, followed the two step popup prompt and you can start to feel the difference in performance; Windows is more responsive and task switching is quicker.
Before you rush out to get your USB stick, do take note that not all flash devices are ReadyBoost capable. Windows requires the device to have a minimum read and write speeds and it will automatically perform a quick test upon insertion of the device. Only if the device pass the test will you be shown the option to use it for ReadyBoost.
Hence to be safe, make sure you check the packaging for confirmation that ReadyBoost is supported before you buy!