If you want to use XSI, ANY dual core will be very useful to you. In 3D applications the rendering performance is about 1.85 times that of the equivalent single core - or in other words your renders will take nearly half the time. I use Cinema 4D & getting a dual core made a BIG difference.
Also, you will want at least mid-range graphics for 3D work, especially if you want to do complex modelling or character animation. 1GB RAM is preferable to 512 as well.
Of course you can probably still do work if you cut back on these but you won't regret paying that bit more. Oh, XSI uses Open GL like all the other main 3D apps I think & this makes NVidia cards a better choice in this case.