On the rear, you can also do a 19" or 20" x 10" with a +42 offset. I haven't pushed the limits on a 10" wide setup but I know a +42 will work and depending on the tire, you can probably push it out a little more (like +40). Also, depending on the wheel design, you can go down to +47 if need be.
On the front, you can also do a 19" or 20" x 9". Again, +42 or so should be good....I wouldn't go much deeper than that...you'll start getting too close to the suspension upright...
And don't forget about brake clearance if you have a big brake kit (well, except for the Wilwood 6-piston setup I'm working on which will have the same caliper clearance needs as the stock caliper.) For those, you'll need a high disk setup...I recommend using a 8.5" wide wheel with a +38 just to be safe...
If you want a comfortable ride and some extra rim protection, the 19" Fat setup is the way to go...ask Cerberus about it...he has that exact setup (and someone else with a GS has it too...I can't remember his name). I'm also going to do this on my SC400.
For that setup, a 19x10 rear is perfect if you're going to use a meaty tire like the S-03...Which, BTW, with the fat setup, you're rather limited to your tire choices but the S-03 is available and that's the tire I'd run anyways...Pirelli is your other choice...but personally, I think Pirelli's SUCK.
Front Wheel - 19x8.5 (+38 to +45) or 19x9 (+42)
Front Tire - 245/40-19