In a word, nothing.
Unless you are a high-end web developer who needs to develop and test ASP
code in a local IIS environment (if the acronyms are unfamiliar, then you
are not), or unless you are connected to a large corporate network with its
extra needs for security, then you do not need XP Pro, and its extra
features will have no impact on you whatsoever.
XP Home is in every way the same Windows code as XP Pro. Byte for byte
the exact same thing. There is no qualitative advantage to running XP Pro
over XP Home, and nothing to be gained. Absolutely nothing. Unless you
actually need or use the specific feature set in XP Pro, then spend your
money on more RAM. You already have Windows XP. Why install the same code on
top of it just to have the name "Pro" on your system?