I beg to differ with JWFokker. XP Pro 64 bit is not built on the XP core, it's built on the Server 2003 64 bit core.
XP 64 bit doesn't have that many instability issues of itself. Driver issues, however, is another matter. Since it's a slimmed down Server 2003 64 bit, it uses drivers for Server 2003 64 bit, and many peripherals (printers, scanners, tablets. cameras) don't have drivers for server OSes.

I prefer XP Pro 64 bit, since it's far leaner and meaner - and faster - than Vista. Then again, I selected my hardware with XP 64 bit in mind, so I don't have driver issues.

Dual core PC - well, it depends on what kind of dual core. If it's a Pentium D series dual core, upgrading to a quad core will significantly increase performance. But if you have an Intel E series dual core (Core 2 Duo), the performance increase will be less - but you can use the same mainboard and RAM.

Keep in mind that buying a standalone version of Vista is expensive! An OEM version can only be sold with accompanying hardware (but two modules of RAM would qualify), and costs less than half of the retail version of Vista.
Same goes for XP Pro 64 bit.

Don't expect the dual core to perform as well as a nicely spec'ed quad core, just because you upgrade to Poser Pro and a 64 bit OS, plus extra RAM. The quad will still be significantly faster at rendering.
Buying a quad for Poser 6 is completely useless, however. Poser 6 is pure single threaded and will only use 1 core.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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