Here's a little comparison:
(Card, Core Clock, Memory Clock)
X800 Pro, 475MHz, 950MHz
X850 Pro, 520Mhz, 1.08Ghz
X1600Pro, 500MHz, 780MHz
All three cards have 12 pixel pipelines. The only edge the X1600 has over the other two is that is has Shader Model 3.0 support (it's almost guaranteed you could overclock the X800 to X1600 core speeds, likely higher; the X800 series is very overclocker-friendly).
Since you're not looking to go PCI-e yet, I'd say go with the X800 Pro. Chances are you'll overclock it to X850 clocks, anyway.