Things to be weary of basically are some programs don't work, there isn't much if any support for 64 bit Anti Virus solutions although Avast Free works fine, there is a Tiny 64 bit firewall which recently came out and fixed lots of problems.... probably doesnt apply to you , but UK ADSL modem users which connects via PPPoA would need to invest in a router as there is no definative drivers yet, ...(thats my predicament)...and for the record I only had an OEM XP Home product key which they accepted and have sent me XP 64bit from Fremont , US without any problems.
Games tend to work fine if not better in my own opinion, and generally when I had it on here before it runs alot smoother, it seems to handle memory tons better for the 1GB plus bracket, you need to be weary of things such as SATA drivers which can be a bit dicey.
Christ, I could go on allday about this lol
Best source on the net right now would be
www.planetamd64.com , there are some die hard XP64 fanatics on there who have an answer for everything gripe you might encounter, but first off...when you install, make sure you have the relevant SATA and Internet drivers arranged and then you can pretty much work from there.
Good luck!