512 is the minimum recommend amount for vista home basic (all the other versions recommend 1gb minimum) but once you start adding a few other things running in the back ground it will really struggle with only 512.
2gb is generally considered the "sweet spot" for vista, but 1 or 1.5GB would improve things very noticeably.
if your not sure what type of memory to buy the link below will scan your system and tell you (you don't have to buy your memory from them, shop around for a decent price but I'd say stick to branded memory)
http://www.crucial.com/uk/index.aspx?cpe=CHAWK uk