if you are running xp then with any less than 256mb of ram it may be a bit slow in some tasks especialy if your processor is also quite slow and you are into video editing or gaming,digital imaging etc.
if you are running low on available memory the computer pages to an area on the hard drive that is then used as ram this is much slower than actual ram.
512mb of ram should see an improvement with xp.adding more than this(if your motherboard supports it)may not see that big of an improvement if you already have 512mb.
latest games benefit from a fast processor and a fast graphics card as well lots or ram.
there are other things you can do such as stop some programs automatically loading with windows and set to manual or disable any uneeded windows services to free up some ram that can be put to better use elsewhere see click here click here