Don't be silly. XP will run quite well on 256MB of RAM, as my PC at home does. I'd prefer a bit more myself, but it's not essential.
I highly doubt that. Depends on your CPU as well. Dont expect to run XP on 256MB's of RAM with anti virus on sheesh, my dad's desktop can hardly run XP Media Center without lagging and its got 512 MB's on a P4 2.8 Ghz CPU and we only have AVG running as well and it still runs slowly. XP needs at least 1 GB to run efficiently with Service Pack 2, otherwise your computer is just going to bog down. XP Pro seems to run fairly well on my laptop with little hang-up's and its only got a 1.8 Ghz AMD Turion mobile CPU and 1 GB of RAM. Although my laptop RAM has better memory bandwidth then my dad's desktop does I believe, mines like 500mhz whereas his is 333mhz, and memory bandwidth is also important in running your OS well.