I'm no computer expert but have always used a Mac laptop. Recently - about 1 year ago - upgraded from the i-Book to a MacBook (Leopard - 1G RAM). Had to get a way of accessing various PC based products, so I got Parallels 4.0 late last year and it was dead slow - until I realised that I was running it on 500K RAM. Just increased the RAM in the Mac to 2G and assigned 1G to parallels.
The difference is unbelieveable! I can now run XP and Mac together with the only cost being a slightly slower Mac. Anyone needing to run both XP and Mac together, I would definitely recommend Parallels - but you need at least 2G RAM.