The maximum official amount of memory (RAM) that the computer can take is 2GB (probably 2x1GB sticks), of type DDR2. Your current 1GB may be made up of 2x512MB sticks which would mean binning them both to upgrade to 2GB. Even if so, I would still do this as its cheap and 1GB is quite tight for Vista.
http://www.crucial.com/uk have a tool that'll automatically tell you what you need.
As requested, here's the "clear english" to what I suspect your problem is
Its not a simple linear relationship between RAM (memory) size and the speed of the computer. The RAM is the computer's
working memory, where it keeps all the things that are currently open, such as Windows, your web pages, email client etc. Think of it like a desk where you can spread your paperwork that you are working on. With a bigger desk, you can look at more paperwork at once.
If you open more applications (or bigger ones) than can fit in your computer's RAM, then this is the equivalent of running out of room on the desk. The computer then has to clear some stuff out and store it temporarily on the hard drive (long term memory), which is much bigger but also
much slower to access - equivalent of taking some of your papers to a filing cabinet down the hallway. If you then switch to the program that it happens to have cleared out to make room, you get a lag while it moves something else out to the hard drive to make room, and then fetches the data on the program back from the hard disk. This process is called
paging (or
swapping to Linux users)
You can prevent this by:
- Opening less programs, web pages, etc at the same time, thus using less RAM
- Buying more RAM (your machine only supports 2GB sadly)
- Switching to a leaner operating system than Windows Vista, as Vista itself consumes RAM when running
- Uninstalling cruft, like all the junk that generally sits in the system tray near the clock, which consumes RAM when its running (especially Norton which seems to consume lots more resources than other antivirus programs)
If you never use more than the amount of RAM you have, there will be virtually no speedup to buying more. On the other case, if you use vastly more than you have and switch between programs often, you will see speedups that are much greater (e.g. could be 10x speedup for only doubling the RAM)
PS Since you are on Virgin Media, you may also notice that the speed may not be consistant and if you run speedtest.net you may get a worse speed at peak times due to congestion on their oversold network. Not that I think that is causing your problems.