Better get it now if you want XP. XP is due off the shelves pretty quick, and that's already the case in a lot of office superstores.
I agree with you wanting XP. Vista is too much trouble, at least right now, however, if you wait a little while longer, either you will have to get a Vista machine or buy a used laptop w/ XP installed (not recommended as used laptops are almost certain to have a fixed life after purchase).
Personally, I would get a MacBook, but those probably will bust your budget. However, the Acer Aspire line (they look soooo 1995, but they run like a top)
There are several model #s out there its hard to keep track BUT
they look like this
...and they come with AMD64 or Intel Pentium processors @ 1.8gHz (I think)
and almost always come w/ 1 gig of ram, sometimes 2
and a 120 gig hard drive. Card readers, a comfortable keybd, and a SuperMulti drive are standard. Starts at $500-600 @staples, watch for sales.