Join Date: Jun 2004
Location: Northern Nevada
Assuming you're a relatively normal-sized person and you don't have a thing for catching big air, the Sirrus should do fine on moderate trails. I wouldn't even buy new wheels, at least at first--get some cyclocross or "29er" tires (it has 700c wheels, right, not 26-inch? If it's 26, you can use mountain bike tires) for the existing wheels and see how you like that kind of riding.
Before you pay for offroad tires, though, check the clearances--not all knobby tires will fit on all bikes. The V brakes give you lots of room, but there may be issues between the fork blades or chainstays.
I have a Rivendell Atlantis, which is a sort of hybrid--Riv calls it an "all around" design--and I've used it for everything from centuries to fire trails with just a tire or tire/wheel swap.
If you do decide to get specific wheels for trails, look for something with 36 spokes unless you're pretty light. Over 160 pounds or so, I'd recommend 36 spokes, and ride with some care--don't be crashing over boulders.