GP4 runs definitly slower if you dont start it with gpxpatch. If you start it with gpxpatch it runs fine.
I own a MSI Starforce 4 TI 4600 which dont support dx9. But i own games that requieres dx9 - like nascar racing season 2003. The game wont start if i havent installed dx9.
Haven't found any problems so try to install it.
But always make a security save point in windows xp. If anything fails or works wrong you simple can go back to your prev. system settings with dx8.
Dont use it with Win9x or ME.
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