RDC works okay, though I've found in most cases VNC is a more robust and extensible solution. Speed-wise they are comparable, with VNC slightly more responsive depending on which server/client configuration you're using.
Both are fairly lightweight, so once you've upgraded those PC's to XP Pro, it is probably worth trying both to see which works better for you. I think RDC is the more 'elegant' solution, but VNC edges it out in performance.
FWIW, I was planning on doing a similar thing with my PC, but ultimately resorted to a second monitor and using Derman's KMRemoteControl. Ultimately I don't mind sharing the keyboard and mouse between machines, but screen real estate is important, so I decided to dedicate a monitor to each.