The only things you will be able to re use in the tower are the case and the drives.

The power supply will need replaced due to age and capacitor swelling, it might very well have killed the video card by putting out "dirty power".

430 watts is not really enough power output in a newer system anyway.

The Q6600 can not be used on a multi cpu board.

DDR2 will not work any any new motherboard worth buying. Newer motherboards will hold 16 to 32 gig per cpu for the most part, but use DDR3 or RDIMM/UDIMM modules.

As far as a decent card for use with Poser, most of the newer cards will work fine for it. Memory is very important in a video card. 2 gig of GDDR5 is what I would recommend.

ATI (Radeon 7850 if your new mobo has PCIe 3.0 or Radeon 6950 with PCIe 2.1) or Nvidia (GTX560) based on your preference.

If you are interested in a multi cpu system there are pleny of entry level priced motherboards and cpu's. Westmere Xeons and motherboards (atx style) that will fit in your case are available. There are Opterons and motherboards that will fit as well. The Xeon based machines will be faster when it comes to rendering due to the number of floating point units in the cpu's. 

Stay away from the 16 core Opterons, they will not work so well for rendering due to a severe lack of floating point units in the cpu. (They have less than the 12 core does) The 16 core Opterons were designed more for I/O operations than number crunching.

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