About drivers, unless your particular model has an option for XP32, you will have to find generic drivers for everything, which may or may not always be available. Many brands use customized stuff and you can only get drivers from the brand's own website/support, where they will refuse to support you if you change the OS (they may even invalidate your warranty altogether) So do extensive googling on the laptop before you buy it.

As for XP 32, you wont be able to use more then about 3GB RAM in any single app. Many apps wont see beyond 2GB. Recent versions of Poser will use ~3GB but become very unstable over 2GB.

Personally i would give Vista 64 a chance. On 4GB RAM and with a fairly recent laptop it runs fast and smooth. The security things mostly go away after you finish installing all your stuff and then only popup when you're trying to do something dangerous, which is not a bad idea. You can also disable them if you really hate them. Poser will also be able to use all 4GB and run more stable.