Ryan: My personal opinion only. 512MB is the minimum that you should consider for reasonable responsiveness using WinXP. I briefly used a tablet with 256MB, and I considered the experience unacceptable.
I also think that 1GB (1024MB) is about the most cost-effective breakpoint between increasing performance and increasing memory cost. Perversely, many vendors sell their 1GB memory upgrades for 3 TIMES the cost of a 512MB upgrade! I think that sucks, but it's their party. I happen to have 1.5GB in my current tablet, but I have to admit that it's probably overkill on my part.
Hope this helps. Good luck!