For those who don't know, Windows XP Professional x64 Edition supports more than 4GB of system RAM. This is a good thing if you have a application that been "ported", written for, 64 bit Windows ...but... pretty much everything available in your local computer store is 32 bit "legacy" software This currently includes Poser. (I hate the term "legacy", since it implies most new software is now obsolete). When running "legacy" software, you'll be burning twice the RAM. For example, if you're using Poser and it was using 2GB of RAM, it'll use 4GB of RAM under XP 64. Not a problem if you have 4GB or more of RAM, but definitely a problem if you don't. (I'm intentionally leaving out the technical details.) By the way, currently Poser will still have the same memory restrictions as the "old" XP as XP 64 until it's ported. Also hardware manufacturers have been slow to jump on board with XP 64, in particular "no names", and older printers / scanners / web cams, etc. From the benchmarks I've seen, "legacy" software (pretty much everything sold today) runs at the same speed, more or less, under XP 64. In theory, performance should be a little worse since it's running under the Microsoft's WOW64 (Windows on Windows 64) translation layer, but it appears to be well-written. This is a first for Microsoft: tight, good performing code. So long and short of it, you're not penalized for performance under XP 64, but expect to use twice the RAM for most applications, until they're ported to 64 bits.