AutoCAD 2008 uses the same DWG format as AutoCAD 2007 and it can read all DWG files from AutoCAD 2 back in the early 1980's.
The API is binary compatible so 2007 applications should run fine or check with your 3rd party developer. 64bit is new so any ARX you want to run on the 64bit version of AutoCAD will require recompiling.
Here are some compatibility highlights from a AutoCAD 2008 Questions and Answers document.
Do AutoCAD 2004/2005/2006/2007 third-party applications work with AutoCAD 2008 32-bit?
Third-party add-on applications based on Visual LISP® or AutoLISP®, VBA, ActiveX® and ObjectARX® programming languages and developer tools for AutoCAD 2007 are compatible with AutoCAD 2008 32-bit. AutoCAD 2008 is binary compatible with 2007 and has the same DWG file format.
Third-party applications for AutoCAD 2004/2005/2006 based on Visual LISP or AutoLISP are also compatible with AutoCAD 2008. Third-party VBA based applications for AutoCAD 2004/2005/2006 are compatible with AutoCAD 2008 32-bit in some cases, but there are instances in which those applications will need to modified. For more details on AutoCAD 2005/2006 VBA-based application compatibility, please refer to www.autodesk.com/developautocad. Third-party applications for AutoCAD 2004/2005/2006 based on ActiveX and ObjectARX will need to be upgraded to be compatible with AutoCAD 2008. Check with your application developer/supplier regarding availability of applications built for AutoCAD 2008.
You can find additional details on migrating applications to AutoCAD 2008 in the Application Migration Guide that is included in the AutoCAD 2008 ObjectARX SDK. You can find the ObjectARX SDK and much more information on developing applications and customizing AutoCAD at www.autodesk.com/developautocad.
Do AutoCAD 2004/2005/2006/2007 third-party applications work with AutoCAD 2008 64-bit?
Third-party add-on applications based on Visual LISP or AutoLISP for AutoCAD 2004/2005/2006/2007 will work on AutoCAD 2008 64-bit. Autodesk has developed an out-of-process interim solution for customers who want to use VBA on 64-bit AutoCAD. Long-term customers will be required to port the VBA applications to VSTA when using 64-bit AutoCAD. Third-party add-on applications based on VBA for AutoCAD 2007 will run out of process on AutoCAD 2008 64-bit. VBA based applications for AutoCAD 2004/2006/2005 may run out of process in some instances but may need to be modified in other instances. For more details on AutoCAD 2005/2006 VBA compatibility, please refer to www.autodesk.com/developautocad.
Third-party applications for AutoCAD 2007 based on ActiveX and ObjectARX will need to be upgraded to work on AutoCAD 2008 64-bit. More information on migrating to AutoCAD 2008 64-bit can be located in the ObjectARX 64-bit Migration Guide included in the AutoCAD 2008 64-bit SDK or at www.autodesk.com/developautocad. Applications for AutoCAD 2004/2005/2006 based on ActiveX and ObjectARX will need to be upgraded to 2007 in addition to being made ready for 64-bit. Check with your application developer/supplier regarding availability of applications built for AutoCAD 2008 64-bit.
Can I run AutoCAD 2008 side-by-side with other AutoCAD platform–based applications?
Yes, AutoCAD 2008 can be installed side-by-side with any other AutoCAD or Autodesk vertical solution—including AutoCAD 2008–based products. These products include AutoCAD Architecture, AutoCAD Map 3D, AutoCAD Civil 3D, AutoCAD Electrical, AutoCAD Mechanical , Autodesk® Inventor™ product families, AutoCAD® Revit® Suite products, Revit® Structure, AutoCAD® MEP, Autodesk® 3ds Max®, Autodesk® VIZ, and AutoCAD LT software applications.
Is AutoCAD 2008 compatible with AutoCAD LT?
Yes, AutoCAD 2008 is fully DWG-compatible with AutoCAD LT 2008. It can read files from all older versions of AutoCAD LT and can save to the AutoCAD LT 2007, 2004, 2000, and Release 14 DWG file formats for compatibility with older products.