I have Mint 6 installed on a Dell OptiPlex GX150 which has a Pentium 3 in it running at 1.1 GHz along with 512 MB RAM. That machine has older specifications than yours, yet I cannot duplicate your exact issue.
However, on that machine, I have Tracker completely removed, no enhanced effects, and use the GNOME menu instead of the mintMenu. I also have a couple of the services and sessions disabled such as Bluetooth and Samba, etc. If my machine will run without any issues, I am am confident that yours will, as well. You just may need to tweak a few things.
This machine was a donation, so I have no idea how XP ran on it. To be honest, I only booted into XP to get the CD-ROM drive to open so I could put in the Mint Live CD. I quickly rebooted (never saw XP, actually), and Mint has been running on it ever since.
While not an exact answer to your post, I know a similar machine with weaker performance will run Mint 6 GNOME.