You cannot have both ATI and nVidia drivers on your PC, they will be competing for the same resources and will conflict with each other. Actually, I heard that you can, but it will take a lot of tweaking.
Since you will not be gaming with this card, who cares how powerful it is? Get an inexpensive as possible ATI video card to fit in the PCI slot.
He's running XP, you can have both drivers on the same PC (rather easily unlike Vista which is an extremely unstable hack breaking WDDM) but it's still a hassle because while you can do it (mainly need to follow proper driver install order), it's just adding headaches and increasing conflicts, which still happens even with one driver and the same hardware just less of them. So the ATi choice since there's already ATi hardware.
I would agree, just get a cheap PCI model, but I would say keeping the same DX/SM/OGL level is a good idea although not required. Although adding an HD2400Pro would likely not hurt things either, and I wouldn't realy say the X1550 has any advantage over it for what you're doing.
If you can find an ati 2xxx series for a similar price, that would be the better choice. You're not going to see a difference in anything at all between the two if you're only doing general app work. But on the other hand, the 2xxx series (and anything above) can process high deff codecs, and is generally more up to date with drivers and power saving technology.