certain OEM laptops wont let you run xp - some will. check the manufac's website and see if its possible.

the 3gb switch is a line you add to the boot.ini in certain versions of windows (xp pro BUT not home, winows server and a few more) to let applications utilise 3gig of ram (if you have 3 gig physically installed) - usually they can only use 2(ish)

sp3 is the lates windows xp update - it apparently, in testing, gave a 10% performance gain - ive yet to see this but it is somewhat faster and smoother.