I can't find a single X1600Pro review to save my life, never mind X1600Pro in crossfire. Without an interconnect I don't know if two X1600Pros will match a X1600XT even. Doubly so in a motherboard without two PCIe 16x slots. Remember that the 'free' SLI uses the PCIe bus to communicate between the cards, and at higher resolutions and frame rates the amount of data adds up quick. So low end cards are probably okay, but as you try to get higher performance out of them, saturating the PCIe bus may get you diminishing performance returns.
The capacity is good, but IMO wasted. It costs extra money, and it's unlikely that SLIing two entry level cards will be a good idea for noise, heat, power use, etc. Two 6600 SLI are not as "good" as a 6600GT, in other words, even though the cost and performance may be similar. Likewise two 6600GT vs 6800GS and so on.
I don't have any benchmarks to the contrary, I just suspect the same holds for the X1600Pro as well.
BTW, if you search forums, I previously opined the X1600XT will become a *very* interesting card once it drops into the low $100s. It's not there yet, but close.
Oh, and the requirements for a rig with two video cards go above and beyond simply a quality PS. A quality 300-400w (read: $30-$40 termaltake or fortron) PS is fine for a non-overclocked 1 cpu with one low to midrange video card system. Once you get to using two GPUs, you have to make sure your PSU is designed for massive wattage on the 12 volt rail. Which gets you up into the 500+ watt 'SLI certified' units (read: $90 and up) if you wish to avoid trouble.