That depends upon the system on which you are willing to dual boot. If you have enough clock speed, memory etc. then answer is you will not loose any performance, because you cann't start both the windows at the same time. Can you???
Windows XP is faster in gaming but you can have problems finding the games compatible with winXP, but if the games you will play are compatible, then you will get better performance in WinXP.
<b><font color=red><i>"All delays are dangerous in war."</b></font color=red></i>