Let's try that again...

Hi renderclipps, I was recently bounced into a system upgrade by a failed motherboard on my old Shuttle 2.5Ghz P4 device, and built one using the CPU and memory you mentioned above. I chose an ASUS P5N32-E SLI 680 board (which is fine but probably not worth the premium) and an nVidia 7900 GS graphics card with 512mb of onboard memory.

It's a great success, and I can recommend the general path you're on. The E6600s overclock really well (via increasing the mainboard bus speed), and I have mine running at 3.15Ghz with no special effort other than an aftermarket heatsink and fan (an Arctic Cooling Freezer 7 Pro - great and cheap at less than £20). The system has been thrashed with extensive testing and is absolutely stable with low core temps. Some people have them running at 3.6Ghz on air cooling, but I'm not going there (boring old fart).

I second the caution re Windows Vista. I'm sure it will turn out to be a fine OS, but it's usually best to let the adventurous find all the problems and let Microsoft work on them for a service release or two.

Regarding memory, be aware that 32bit operating systems have quite a low limit on how much they can use. With Windows XP and Vista it's usually a max of about 3.4GB or so. And applications running under 32bit Windows can only access about 2GB. To use 4GB or more effectively you'll need to use a 64bit operating system (there are 64bit versions of XP and Vista). But the 'problem' with 64bit Windows is that Microsoft only allows approved drivers, so getting older peripherals (printers, scanners, etc) to work may be impossible 'cos manufacturers aren't willing to support older products for a small market. Plus very few applications are written native for 64bit yet, so they have to run under 32bit emulation. 

2GB, or maybe 3GB, of RAM will be sufficient for hobbyist and semi-pro work for some time.

You don't need cutting edge graphics cards for Poser and Photoshop, and I wouldn't be surprised if you couldn't tell the difference in those programs between your Geforce 6200 and the latest 8800 effort. I had to buy a new card as my previous GeForce was AGP, and went for the 512mb 7900GS because it was cheap and I use Hexagon and occasionally light game (though am hopeless). nVidia cards are often mentioned in preference over ATI because of supposedly superior OpenGL drivers - which is what 3D users want for their work windows. Though I'd bet that an ATI card would be fine. Fancy cards might need lots of power too, so make your case power supply is up to the task.