wespose: I advise you to wait and save up for some more power. That is a low end machine nowadays.

For Poser 7, I'd recommend the following:
Intel Core 2 Quad CPU (Q6600 or better, but even the cheapest quad outperforms the 4400 by a factor of 2)
4 GB DDR2-800 (2x2 GB, so you can upgrade to 8 GB when desired)
nVidia 8500 GT or better for graphics. Definitely NOT ATI, their OpenGL is still wonky, and forget about 64 bit support.

Bryce will also be quite happy with a more powerful CPU.

Stick to XP. Vista consumes far too many resources all by itself. Windows 7 will be better (resource usage comparable to XP, according to a beta tester), but it won't be here until late 2009.

XP Pro 64 bit will run fine on a machine as described above, but printer drivers, scanner drivers, tablet drivers and wireless network drivers might be troublesome - first check hardware support!
Why XP Pro 64 bit? Since Lightwave and Poser will really enjoy the extra memory space it provides: Vista 32 and XP 32 only give you 2 GB per application, XP 64 bit will give 128 GB of (virtual) memory per application - if the application can make use of it. Poser 7 can use up to 4 GB on a 64 bit OS, Lightwave 7 might be able to use the full 128 GB of address space, for Bryce it doesn't matter, and for Photoshop 7 it doesn't matter either.

Again, don't buy ATI: that'll rule out the XP 64 bit option. And you're as good as hosed if you want to run Linux too.

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

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