don't be misled by the above link, you need atleast dual core processor like AMD X2 and 2 GB RAM to run any application smoothly. get a separate graphics card (geForce 8600GT is cheap these days) if you plan on using only vista (i.e. no XP). my PC (config in my sig) runs vista but is not able to run any game even at 640x480 in vista, whereas the same game runs at 1024x768 in XP.
me with c2d e4500 2.2GHz, 512x2 @ 800MHz RAM, intel gma x3000 onboard gfx.
vista runs real smooth.
i play GTR2 at 1024 @85Hz, full gfx., that game slows only when there are more than 25 cars on circuit

cod4, played it on latest gfx. driver at 1024, med. gfx.
battlefield2 at 1024 @full gfx...
@OP, if you're getting Intel m/b, then get the one which has gma x4500 (G43 chipset)
X4500 supports DirectX 10 and Shader Model 4.0 features. Intel designed the GMA X4500 to be 200% faster than the GMA 3100 (G33 chipset) in 3DMark06 performance and 70% faster than the GMA X3500 (G35 chipset).
but know this, all the onboard gfx. will use the CPU a lot, so get a good processor too. like core 2 duo