You will only notice it if you do alot of memory intensive tasks. Maybe Video editing, Rendering, 3D graphics work etc. But for normal tasks like word processing, spreadsheets and internet. You'll see no difference what so ever.
I have already got 1g of ddr in xp home.I use my puter for video editting,and upgraded from 512mbs to 1.5g but found the three sticks of ram were incompatible with each other.I then changed my ram to all from the same mfg and XP threw all kinds of wobblies.I removed 1 512mb stick and XP has run smoothly since.Only time I find XP needs loads of RAM is when burning a DVD from an image at 2X,set it off at 4X and the RAM usage dropped?(700mbs used while burning at 2X,but only 50ish mbs at 4X))