Well, it's been a while, again. :) I was a regular on this forum from 2004-2007 and made a slight return in 2009 when I was mixing an album by the Swedish band Bibleblack. That was the last one in a row of metal albums passing through my DAW, and by that time I realized I hated it. So I quit. :) Sold off most of my hardware and uninstalled most of the software. But here I am again, JUST FOR FUN this time! I've re-installed Sonar (almost 10 months ago) and some old plugins that I know from the "before-time", and I've started to learn everything from the bottom, basically. Thing is, I'm still on Sonar 8.0.2. As far as I can tell it's a stable system. I'm running the 32-bit version under 64-bit Windows 7 (due to the fact that the 64-bit Sonar did NOT like most of my old plugins, and I need them to live). But I need some hard facts here: - Will a upgrade to 8.5, X1, X2 or whatever result in a performance gain and/or increased stability? I rarely use the included plugins or instruments, I prefer to use the ones I know from before, so that's not a selling point that works on me. :) I have a quad core system, and I can tell that Sonar utilizes all four cores in a great way. The load is even between the cores. I haven't had a single Sonar crash since I started running it in the current configuration (x86 on x64), besides a "crash on exit" that happened ONCE. So to summarize: should I upgrade or stay at 8.0.2? Thanks!!

You can not get the upgrade to 8.5 anymore.
AFAIK you should have the 64-bit version of 8.02 on your install DVD's and try that. You could install the 64-version beside the 32-bit version and compare. At least you can utilize more RAM that way.

I can't comment on X-versions as I don't have them.
 
EDIT: Oops. You tried the 64-bits already. Well, it's up to you how much you value the non-64-compatible VSTs.

So maybe I'll just stick with the x86 8.0.2 version for now. After all, it works.

If it ain't broke...

64-bit SONAR won't improve performance or stability. It just buys you access to more memory, assuming you have more than 4GB installed. More RAM is a big deal for sampled instruments, but if you're recording bands (read: real audio) then there's not much benefit. Stick with 32-bit SONAR and continue to enjoy all your familiar plugins without hassles. 

As you noted, 8.0 knows what to do with 4 cores. The biggest drawback you're likely to encounter is that 8.0 had issues with delay compensation on external effects, and that's only going to be a problem if you use outboard gear such as hardware reverbs or compressors.

If your software has all the features you need and is stable then I would not upgrade. 
As said, you can install multiple versions of Sonar and try them out and see how it goes. 
I tried the trial version of X1 for a month and saw no need to upgrade so I carry on with what I have  without issues. 

If your like me and record mostly real audio then you don't need a lot of memory. It's VST's that need you to go to a  64 bit system.  

Hey, thanks guys! I have only 4GB RAM and I run 95% audio tracks with lots of VST effects. Sometimes an occational VSTi synth. I'll stick with v8 for now.

You can only use just shy of 3 Gigs of RAM under 32 bit. 
But if it's not jamming up then your fine. I found I could not use Session Drummer with the 32 Bit system and 2 Gigs of RAM. 

My experience is 64 bit sonar is a performance increaser over the 32 bit version. That could be specific to my system / audio card or the 64 bit version of some plugs. It's quite evident here though, same project(s) run at lower latency settings without pops/clicks in 64 bits. 

Bitflipper is right, no need to upgrade especially if you aren't sold on the new features of X1/X2. More and more VST makers are making 64bit versions of their plugins so I do think that eventually you will want to go 64bit.

I will say that I have had allot less problems recording full live bands for hours on end on X1 than Id experienced trying the same thing with 8 & 8.5.

No dropout probs, problems with pops, crackles, etc... are non existant in 64 bit now,  YMMV

but for your needs with your plugins, your better off staying with what ya have just for the fact that you have them, know them and use them.

Like Bit said, "If it aint broke..."