I currently have Sonar Home Studio 6 XL running on a desktop XP machine.  Well, my cheap, piece of junk, Phonic Helixboard 18 Firewire mixer fried out and I bought a Presonus AudioBox 1818VSL USB interface.  I have a Win 8, quad core, 64 bit laptop that my Presonus links up to very nicely.  If I purchase the new Sonar X3 Studio, will I be able to fully access all my Sonar 6XL files I have stored on my external hard drive?  Also will Sonar 6XL Home Studio run on a Windows 8 machine?  That, I highly doubt, but I just thought I'd ask.

The only problem may be the plug-ins. If the projects use 32bit DX plug-ins, 64bit SONAR cannot access the plug-ins. They will open OK in 32bit SONAR (which will run on 64bit Windows) provided the plug-ins are properly installed. Some of the old DX plug-ins are no longer included in X3 so it will be necessary to perform an advanced install of SHS to get the plug-ins on the new machine. IOW, you may need to install both 32 and 64bit X3 and the plug-ins from SHS. Since you will need the plug-ins from SHS, it may be worth a shot to install the whole thing. Without any direct experience with SHS on Win8 I have no idea how well it will run.

As  said, the thing that really changes is the jump from 32 bit OS to the 64 Bit world. Everyone here has had to go through this over the last couple of years so your not alone. So there is plenty of help sorting out the odd issue in the X series forum which is the main forum. As  cook said, mostly it's plug ins that change. The software still maintains the same stuff it always did, record audio and midi. Lots of big improvements so you'll be happy with those changes.
Try the free demo first if your not sure. It will take at least a week to feel comfortable again.
I was also going to mention it's a good idea to have all your old projects saved as CWP files and not Bundles. CWB files are often trouble makers when opening them in the newer versions. But CWP files will always open, even if it has to be in Safe Mode. 
 
I've opened old ( 2006) Home studio files without too much trouble in X3 Studio. They are all CWP files with standard issue Cakewalk plug ins used. It will just warn you that they might sound different. 
I've had more issues with projects that were recorded at a different clock rates. Those the audio tracks will be blank until you convert them. 

Ok , I've installed the 64 bit version and like you said no plug ins. So your telling me if also install the 32 bit version, most of the plug ins will be there? The only plug ins I've ever
used are EQ, compression, reverb, & delay. All cakewalk effects. So then I would just use the 32 bit version to access my old recording projects and I can use the 64 bit version from this point on? I really do like X3. Its fast on my Win 8 laptop.

32bit DX effects do not run in a 64bit host without first wrapping the plug-ins in a DX-to-VST wrapper (not included with SONAR) then loading them via BitBridge. Even then, some do not work correctly. The best way to work with projects containing 32bit DX plug-ins is to use a 32bit host. Based on the info here many of the older 32bit DX effects are no longer bundled with SONAR.  Run the advanced install option for SHS to get the older effects on the new PC.
 
Then use the 32bit version to open old projects and the 64bit version for new work. X3 is a big step up from SHS.