I have an AGP X1600 Pro 512 (to hold me until early 2007 when I rebuild for Vista and DX10 cards.) A x1800 should be much better. Now that the price is low for the x1600/1650 it is a good deal though. OR a 7600GT, or a 6800GS if you can find one. I wouldn't get anything faster than that in your time frame unless don't plan on getting a DX10 card until spring/summer 2007 when the entry and mid-range cards arrive and the first price drops happen.
Don't buy either of them The X1900 is compatible with GPU acceleration which will multiply the performce of your cpu by 5 times. The x1950 pro has this ablity that neither of the other two cards have. WAIT. The X1950 Pro will come down in price after Christmas . Here is an article from Stanford University about what you can do with GPU acceleration http://folding.stanford.edu/FAQ-ATI.html. this design is most effective with the AMD opteron because it uses HTT. on Intel you have a northbridge in the way. This is the first variation of AMD's Torrenza archtecture which will render Dual and Quadcore obsolete because they are too energy inefficient for large uses and the measly 150,000 cpu's that go into the enthusiast market will not support the engineering and manufacturing expense of these designs.
I retired in June as director of engineering at Sandia Nati0onal Labs which provides the supercomputer support for Los Alamos National Labs. The future is single core cpu's with accelerators either GPU's or dedicated Accelerators like the IBM Cell chip. http://www.supercomputingonline.com/article.php?sid=118...
In 64 bit operation like Vista these pairs are 2.5 times faster than any quad core and 5 times faster than a dual core.