Upgrading graphics cards

So, I'm planning on building a PC. I don't really have a budget to worry about but ... The choices are an ATI X1600 or an ATI X1800. I know you'd say "X1800!!1 Zomg" But is it really necessary for a few more frames and a bigger card blah blah~?

Specs for the pc (I'll be sure to be specific):

AMD Athlon 64 X2 4400+ Toledo 1GHz HT Socket 939 Dual Core Processor

Patriot Signature 2GB (2 x 1GB) 184-Pin DDR SDRAM DDR 400 (PC 3200 Unbuffered System Memory

ASUS Atlas A-45GA ATX12V 2.0 450W Power Supply

ASUS A8N32-SLI Deluxe Socket 939 NVIDIA nForce SPP 100 ATX AMD Motherboard

And a PC3-426BK Plus, Black (Koolance Integrated Liquid Cooling system with Lian Li case. :P)

Stacking up to approximately $1,500

Why are you buying an SLI-Ready Motherboard if you aren't going to be running SLI?  Wasted money, in my eyes.
I agree with the above statement.  I would reccomend looking at nVidia and all that before you make the choice to upgrade.  Also, you might be interested in knowing that when DirectX 10 comes out (which will be simultaneous with Vista), your graphics card will have to run under an emulation mode, because DX10 will have no backward compatability, and there is currently no card that supports DX10.
Just keep those in mind.