Vista Aero is not directX 10 based first of all, all it required is a DirectX 9c complient graphics card, even a geforce fx 5700 which is over 4.5 years old will run it perfectly.
DirectX 10 brings other enhancements such as a new Driver model, ability to reset the graphics adapter on the fly when required, user mode drivers & most important a very high efficiency lavel.
Even if u don't have a directx 10 card, u will miss nothing with vista & a DX9c based card, although u won't be able to play games, but not everyone playes games. Onboard Radeon X200 is equivalent to geforce fx 5200 in performance, it might be slow in UI drawing at resolutions above 1600X1200, but otherwise, it fine for the job