HP XW6200
Intel Xeon 3.0 Dual Core, 1GB Ram
Currently uses GeForce6600GT 125mb Pci-Express slot

Need to upgrade to a better card. money is no object. High 3D rendering is a must.

PS - would this meet my needs?
SAPPHIRE 100154L Radeon X1800XT 256MB.

ok you said money was no issue so i wont hold back

first off if its for gaming you are going to want to purchase a 7900gtx

if its for video editing, graphics, blah blah get an ATI x1900xtx

now if money is actually an issue then just take the x off the end of both of those cards and you should be good

If money is no object, then either a 7900GTX or X1900XTX will give you an awesome increase over your current card. The X1800XT is nice, but if you have cash to burn then go with one of the flagship cards. Don't let anyone tell you that one is better than the other, especially for gaming; you'll have to look at all the benchmarks and decide yourself. Keep in mind that the ATi card is more of a power hog, especially when idle.

If money really is no object then get 7900 GTX-XXX-XXL edition or 1900XTX-XXX-XXL edition.

If not go with a 7800, lesser 7900 or lesser 1900 or 1800.

By 'High 3D rendering' do you mean gaming or actual 3D rendering - in which case scrap all of the above and look for a Quadro or FireGL.

Since this is an XW workstation I assume this is for rendering, I would have to agree that you should go with a high end FireGL or Quadro, gaming cards may be acceptable for low end rendering but if you are looking at high end I would look at the newest FireGL ( I believe 1GB on card memory is the max...)

You may want to ask users of your particular program for their experience (i.e. Autodesk hardware forum's)

I think the best long term buy would be the RADEON X1900XT or XTX due to more advanced core (AA+HDR).

It is soooo nice.

If money is not an issue and you want to do render.
Get an Quadro 4500 X2 and gelato hardware render you can use the graphics card to acelerate the render and get a good card to work in viewport.

With it you get the equivalent to 2x7900gtx in Direct X and the fatest thing to can get in OpenGL.

If you work in video edition or similar get an ATI 1900xtx.

Note: The xw6200 has Intel chipset and only 1 PCIe 16x. You can't go Crossfire or SLI.