I was in your situation exactly. I own an RME card and having read about the potential problems I was quite apprehensive about installing GS3 especially since I compose for a living and couldn't afford any down time.
The good news for me is that it worked without any problems at all. (I did receive an error message "The procedure point entry.....could not be located...) but I have not had any problems yet. Actually I have a MOTU micro lite as a midi interface and right after the install, Giga would not see any of those midi ports but did see the RME port (I did not do anything in the registry before the install) but after reboot my micro lite ports showed up. Phew!! I was relieved. Good Luck.
p.s. I tried to load an old GS2 setup...no problems.
I'd only install GS3 in the middle of a deadline on a fresh system HD. You have to uninstall GS2.5 before installing GS3, and you probably don't want to lose GS2.5 right now.
If we ignore the time that it would take to format a drive and install windows and all the required drivers - or to ghost your existing drive and remove GS2.5 - the install time of GS3 is really fast - like a minute or two. Copying the content takes much longer. You'll want to copy at least the content from the application disc to get the impulses. Add more time for VGS and the other goodies. Then there's some setup for ports and stuff. You can ignore registration for 10 days.
Lunch? Not unless you eat really, really slowly...
Yes, be nervous. But how could you resist? I've wasted many hours in my 7 days of use so far, revising setups, changing things, tracking down configuration problems and possible bugs. I got the message during installation "GLJ2.tmp Entry point not found The procedure entry point ?GetMessageMap@CEWRegion@@MBEPUAFX_MSGMAP@@XZ could not be located in the dynamic link library EWCTL32.dll" .... and ignored it, like any good musician. Still don't know if it mattered, but I copied down the message just in case.
Stacking? Check out the Larry Seyer drums. Each drum/cymbal is its own instrument. Stack them and build a kit. Set the volume, pan and whatever of each drum separately. Don't like the snare? Remove it and add a different one.
I was hoping to use it in this way and thought I would use MIDI rules to make it work. It's easier than that. Just stack and go.
BTW, check out the mod wheel on the toms. You can move your stick from the center to the edge to the rim.
The drums sound good dry. They sound killer with the associated impulse files.
Don't have too much fun though. I hear that you have a project to complete.
More than anything (besides added stability and voices) - I love the stacking feature !!!
Likewise, I had no problems at all. Had it up and running in about 1 hour, which wasn't bad considering transfering all of the samples took the longest. I bought Kontakt 1,5 at the same time and I am finding Giga 3 easier to learn so far.