Ive never had any problems with SU and vista, and you don't often hear aobut them, its usually more to do with graphics drivers and (intellipoint) mouse software π
In terms of purchasing a laptop for SU stuff, the things to look out for are a dual core processor with a highish clock rate (2.4 ghz or thereabouts) and a proper graphics card. Of the two the graphics card is probably the most important, lots of laptps use integrated graphics to save money, but these dont work with SU.
If you can, go for a quadro graphics card as they generally have the best drivers for supporting openGL (the thing SU uses)
Its also very important do shop around, as different places have deals on all the time, and dont feel presured to buy a computer, i can sometimes be worth waiting for a it to try and get the best deal possible.