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Upgrading a x50v to wm5 or wm6 will give you a speed reduction, but if your only using it for GPS then you should be okay.
There is or used to be GPS software by Tomtom that ran fine on wm2003se.
Its called Tomtom Navigator.
If this software is anything like whats preloaded onto Tomtom GPSs, then it is great!
But if you already own Garmin XT software, which requires wm5+, then you would need to upgrade.
You may be able to find a wm5 cd on ebay or something, or check out
xda-developers - Powered by vBulletin for homemade wm6 roms.
You should probably get a new memory card if you plan to use GPS. You can still use your microdrive, but in 2009 microdrives are obsolete and flash based cards, which use less power are now a lot cheaper. Check out Newegg for CF Cards.
If you want to get a sd card, realize that 2003se doesnt work with sdhc cards, which is most 4gb+ cards avaliable today. Wm5 has some sdhc compatiblity and wm6 has full sdhc compatiblity.
Hope that helps...