Open source alternative to ArchiCAD for ubuntu 8.10 x64

[madanabhat27 at gmail.com]

Hi folks ! 
I deal with a lot of CAD and Building Information Modeling coz I design a lot of houses and other such buildings as I am an architect and as I would have probably told you guys in my last email I have just migrated from MS Windows Vista Business x64 and I would like to know weather or not there are any open source alternatives for my trusty ArchiCAD which are compatible with Ubuntu 8.10 x64 and if so then how much do they cost ? 
Yours , 
KMB
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