Minor fixes needed, any help?

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So im on snow leopard 10.6.6 (shortly to update to 10.6.7) and I can boot from hdd and use whatever screen resolution, both of which took me a while to solve.

 

Now I have some minor problems that I need to fix;

Firstly my ram doesnt show up correctly, I have 4gb 1600mhz g.skill ripjaws (ddr3 of course)

but it shows up as 4gb ddr2 800mhz?

 

Also i cant install xcode, it just says installation failed, and xcode is the sole reason I am trying to run mac os x so help here would be most greatly appreciated.

 

Also is there any way to get my 6870 recognised and running flawlessly?

 

Thanks in advance

 

system spec;

asus p8p67

i5 2500k

 

also if anyone could let me know if there's a way to get either my 6870 hdmi audio working or if theres a kext for the asus xonar dx soundcard that would be great