My suggestion is, if you want Pro, wait until you purchase the computer to get it, if you're buying from a retail OEM (Dell, HP, Lenovo,etc.). The OEM's will burn you through the seat of your pants to install Pro, I've seen them for as much as $200 extra.
In the US, you can go to Windows Anytime Upgrade in the Start Menu, click onto it, Pro costs $89, Ultimate $139 (plus taxes). My upgrade to Pro cost less than $97 after taxes. It's a 10 minute process, your files stay the way they were, it'll reboot a time or two, and you're done.
I originally upgraded to Pro for the XP Mode, but since I have a retail copy of XP Pro, I use VirtualBox instead. I don't like the fact that Windows Virtual PC doesn't support Linux as much as VirtualBox does, either. Also, the guest additions of Windows Virtual PC won't work with Win 2K.
So while there's advantages to Pro (you get XP Pro SP3 in addition to Pro), I don't care for the "preconfigured" environment that it has. However, I concede that for those who needs it, it's a deal that's hard to beat.
I just don't think it's worth $200 extra for the OEM or the sales floor to install it for you. Educate your self on your options before buying, and plan it out, you'll save possibly $100 by doing so.
Cat